Fix string literals immediately followed by line continuation

Previously:

```f90
print*, 'this looks like implicit '&
     'concatenation but isn''t'
print*, "this explicit concatenation "&
     // "is intended"
```

would get parsed as

```
8:2   - 9:33        print_statement
8:2   - 8:7           "print"
8:7   - 8:8           format_identifier
8:7   - 8:8             "*"
8:8   - 8:9           ","
9:7   - 9:33          output_item_list
9:7   - 9:33            string_literal `'concatenation but isn''t'`
10:2  - 11:23       print_statement
10:2  - 10:7          "print"
10:7  - 10:8          format_identifier
10:7  - 10:8            "*"
10:8  - 10:9          ","
11:7  - 11:23         output_item_list
11:7  - 11:23           concatenation_expression
11:7  - 11:7              left: string_literal
11:7  - 11:9              "//"
11:10 - 11:23             right: string_literal `"is intended"`
```

Note first half of literal is missing!

Now gets parsed as:

```
8:2   - 9:33        print_statement
8:2   - 8:7           "print"
8:7   - 8:8           format_identifier
8:7   - 8:8             "*"
8:8   - 8:9           ","
8:10  - 9:33          output_item_list
8:10  - 9:33            string_literal
8:10  - 8:39              `'this looks like implicit '&\n`
9:10  - 9:33              `       'concatenation but isn''t'`
10:2  - 11:23       print_statement
10:2  - 10:7          "print"
10:7  - 10:8          format_identifier
10:7  - 10:8            "*"
10:8  - 10:9          ","
10:10 - 11:23         output_item_list
10:10 - 11:23           concatenation_expression
10:10 - 10:40             left: string_literal `"this explicit concatenation "`
10:40 - 10:41             "&"
11:7  - 11:7              "&"
11:7  - 11:9              "//"
11:10 - 11:23             right: string_literal `"is intended"`
```

Full literal text is now included.

No tests because this is in the CST, and that's not captured in the tests

Author: ZedThree

src/scanner.c

@@ -153,13 +153,13 @@ static bool skip_literal_continuation_sequence(TSLexer *lexer) {
         return true;
     }
 
-    skip(lexer);
+    advance(lexer);
     while (iswspace(lexer->lookahead)) {
-        skip(lexer);
+        advance(lexer);
     }
-    // second '&' required to continue the literal
+    // second '&' technically required to continue the literal
     if (lexer->lookahead == '&') {
-        skip(lexer);
+        advance(lexer);
         return true;
     }
     return false;
@@ -365,6 +365,7 @@ static bool scan_string_literal(TSLexer *lexer) {
             // the end of the literal. We also need to check that an
             // escaped quote isn't split in half by a line
             // continuation -- people do this!
+            lexer->mark_end(lexer);
             skip_literal_continuation_sequence(lexer);
             if (lexer->lookahead != opening_quote) {
                 return true;