Fix whitespace issues with block directive formatting (#177)

Issues:
* top-level/starting block directive has unecessary preceding newline
* top-level block directive is unexpectedly indented
* nested block directive child has unecessary newline before "}"

Example snippet:

```swift
import Markdown
print(Document(
    BlockDirective(name: "Metadata", children: [
        BlockDirective(name: "TitleHeading", argumentText: "Example")
    ])
).format())
```

Expected output:
```
@metadata {
    @TitleHeading(Example)
}
```

Actual output (before this commit):
```

    @metadata {
        @TitleHeading(Example)

    }
```

Author: mportiz08

Sources/Markdown/Walker/Walkers/MarkupFormatter.swift

@@ -463,7 +463,8 @@ public struct MarkupFormatter: MarkupWalker {
                 } else if let numeralPrefix = numeralPrefix(for: parentListItem) {
                     prefix += String(repeating: " ", count: numeralPrefix.count)
                 }
-            } else if element is BlockDirective {
+            }
+            if element.parent is BlockDirective {
                 prefix += "    "
             }
         }
@@ -1105,7 +1106,9 @@ public struct MarkupFormatter: MarkupWalker {
     }
 
     public mutating func visitBlockDirective(_ blockDirective: BlockDirective) {
-        ensurePrecedingNewlineCount(atLeast: 1)
+        if blockDirective.indexInParent > 0 {
+            ensurePrecedingNewlineCount(atLeast: 2)
+        }
         print("@", for: blockDirective)
         print(blockDirective.name, for: blockDirective)
 
@@ -1124,13 +1127,13 @@ public struct MarkupFormatter: MarkupWalker {
 
         if blockDirective.childCount > 0 {
             print(" {", for: blockDirective)
+            queueNewline()
         }
 
         descendInto(blockDirective)
 
-        queueNewline()
-
         if blockDirective.childCount > 0 {
+            queueNewline()
             print("}", for: blockDirective)
         }
     }

Tests/MarkdownTests/Visitors/MarkupFormatterTests.swift

@@ -376,6 +376,18 @@ class MarkupFormatterSingleElementTests: XCTestCase {
         )).format()
         XCTAssertEqual(expected, printed)
     }
+
+    func testPrintBlockDirective() {
+        let expected = #"""
+        @Metadata {
+            @TitleHeading(Example)
+        }
+        """#
+        let printed = BlockDirective(name: "Metadata", children: [
+            BlockDirective(name: "TitleHeading", argumentText: "Example"),
+        ]).format()
+        XCTAssertEqual(expected, printed)
+    }
 }
 
 /// Tests that formatting options work correctly.
@@ -1533,10 +1545,65 @@ class MarkupFormatterTableTests: XCTestCase {
         """
 
         XCTAssertEqual(expected, formatted)
-        print(formatted)
 
         let reparsed = Document(parsing: formatted)
-        print(reparsed.debugDescription())
         XCTAssertTrue(document.hasSameStructure(as: reparsed))
     }
 }
+
+class MarkupFormatterMixedContentTests: XCTestCase {
+    func testMixedContentWithBlockDirectives() {
+        let expected = [
+            #"""
+            # Example title
+
+            @Metadata {
+                @TitleHeading(example)
+            }
+            """#,
+            #"""
+            @Tutorials(name: Foo) {
+                @Intro(title: Bar) {
+                    Foobar
+                    
+                    @Image(source: foo, alt: bar)
+                }
+            }
+            """#,
+            #"""
+            # Example title
+
+            @Links(visualStyle: list) {
+                - ``Foo``
+                - ``Bar``
+            }
+            """#,
+        ]
+        let printed = [
+            Document(
+                Heading(level: 1, Text("Example title")),
+                BlockDirective(name: "Metadata", children: [
+                    BlockDirective(name: "TitleHeading", argumentText: "example"),
+                ])
+            ).format(),
+            Document(
+                BlockDirective(name: "Tutorials", argumentText: "name: Foo", children: [
+                    BlockDirective(name: "Intro", argumentText: "title: Bar", children: [
+                        Paragraph(Text("Foobar")) as BlockMarkup,
+                        BlockDirective(name: "Image", argumentText: "source: foo, alt: bar") as BlockMarkup,
+                    ]),
+                ])
+            ).format(),
+            Document(
+                Heading(level: 1, Text("Example title")),
+                BlockDirective(name: "Links", argumentText: "visualStyle: list", children: [
+                    UnorderedList([
+                        ListItem(Paragraph(SymbolLink(destination: "Foo"))),
+                        ListItem(Paragraph(SymbolLink(destination: "Bar"))),
+                    ]),
+                ])
+            ).format(),
+        ]
+        zip(expected, printed).forEach { XCTAssertEqual($0, $1) }
+    }
+}

Tests/MarkdownTests/Visitors/MarkupTreeDumperTests.swift

@@ -95,7 +95,6 @@ final class MarkupTreeDumperTests: XCTestCase {
         └─ HTMLBlock @42:1-42:90 #71
            <!-- Copyright (c) 2021 Apple Inc and the Swift Project authors. All Rights Reserved. -->
         """
-        print(everythingDocument.debugDescription(options: [.printEverything]))
         XCTAssertEqual(expectedDump, everythingDocument.debugDescription(options: [.printEverything]))
     }
 }