Fix the patterns for code blocks and the handling of newlines when matching patterns (#9921)

* Fix the patterns for code blocks and the handling of newlines when matching patterns

TextMate grammars are matched by-line and therefore can never match `\n`. However our Dart implementation allowed this, which resulted in some bugs when code blocks are not closed.

This updates the grammar to instead use $ to match the end of the line, and updates the parser to only ever match on the current line and not consumer the newline. With this change, the result matches the output of the TypeScript implementation used for testing in Dart-Code.

The source PR for this version of the grammar is at https://github.com/dart-lang/dart-syntax-highlight/pull/91 (I'm filing these together because one repo owns the syntax and one owns the parser).

See Dart-Code/Dart-Code#6130
See Dart-Code/Dart-Code#6131

* Update NEXT_RELEASE_NOTES.md