fix: first node in (line) captures whitespace

Problem:
Whitespace is still captured in the first node of a line. #23

Solution:
Avoid regexes that explicitly capture leading whitespace. The `extras`
directive makes this unnecessary.

fix #23

Author: justinmk

corpus/codespan.txt

@@ -40,9 +40,11 @@ an error`.
         (word))
       (word)
       (word)
-      (ERROR
+      (codespan
         (word)
-        (word))
+        (MISSING "`")))
+    (line
+      (word)
       (word))))
 
 ================================================================================

corpus/heading3-column_heading.txt

@@ -236,6 +236,13 @@ NOT column_heading
 tilde ~/foo/bar foo~ bar
 trailing-space-is-NOT-column_heading ~ 
 
+buffer-diagram
+|-----|
+|foo  |
+|~    |
+|~    |
+|-----|
+
 
 --------------------------------------------------------------------------------
 
@@ -248,4 +255,26 @@ trailing-space-is-NOT-column_heading ~
       (word))
     (line
       (word)
-      (word))))
+      (word)))
+  (block
+    (line
+      (word))
+    (line
+      (taglink
+        (word)))
+    (line
+      (word)
+      (ERROR
+        (word))
+      (word))
+    (line
+      (word)
+      (word)
+      (word))
+    (line
+      (word)
+      (word)
+      (word))
+    (line
+      (taglink
+        (word)))))

corpus/line_block.txt

@@ -53,7 +53,7 @@ text text text text
       (word)
       (word)
       (word))
-    (MISSING "help_file_token1")))
+    (MISSING "<")))
 
 ================================================================================
 blocks: with blank line at end of document

corpus/optionlink.txt

@@ -136,10 +136,12 @@ x `after_codespan`'s
       (argument
         (word))
       (word))
-    (ERROR
+    (line
       (word)
       (codespan
-        (word)))))
+        (word))
+      (word
+        (MISSING "'")))))
 
 ================================================================================
 NOT optionlink 3 (FIXME)

grammar.js

@@ -21,7 +21,7 @@ module.exports = grammar({
   rules: {
     help_file: ($) =>
       seq(
-        repeat(/[\t ]*\n/),  // Eat whitespace at top of file.
+        repeat($._blank),  // Eat blank lines at top of file.
         repeat($.block),
       ),
 
@@ -67,8 +67,8 @@ module.exports = grammar({
       seq("'", token.immediate(/[^'\n\t ]*[^'a-z\n\t ][^'\n\t ]*/), token.immediate("'")),
       // NOT optionlink: single char surrounded by "'".
       seq("'", token.immediate(/[^'\n\t ]/), token.immediate("'")),
-      // NOT taglink: "||".
-      /\|\|*/,
+      // NOT taglink: "||", "|"
+      /\|\|+/,
       '|',
       // NOT listitem: "-" or "•" followed by tab.
       /[-•]\t/,
@@ -125,7 +125,7 @@ module.exports = grammar({
     )),
 
     // Lines.
-    _blank: () => field('blank', /[\t ]*\n/),
+    _blank: () => field('blank', '\n'),
     line: ($) => choice(
       $.column_heading,
       $.h1,