Syntax highlight confusing in unicode string formatting

[tgarm]

When syntax highlight a unicode string, the syntax highlight does not very correct.
For instance, to format a value val in string may look like:

• Without bracket
  

This seems will format val世界 instead of val.

• With bracket
  

This shows brackets are not needed.

It seems confusing how to correctly format val in this string in this syntax highlight.

[DanTup]

Thanks for the report! There are quite a few issues with the current syntax highlighting. The goal is to fix them when adopting server-provided highlights through LSP (in #2202) since the server will produce reliable highlights using the real Dart parser, rather than using regex.