VS Code: Semantic highlighting

[lars-reimann]

Is your feature request related to a problem? Please describe.
Template strings are currently fully rendered with a single color, even the template expression.

Describe the solution you'd like
Template expressions should stand out, which requires semantic highlighting.

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• LSP4J used to have an unofficial way of providing semantic highlighting, which is now removed
• This was also removed from Xtext
• LSP4J supports Semantic Tokens (LSP v3.16)
• It does not yet seem to be supported by Xtext