feat: let TokenSelector support multi-languages

[Colibrow]

Related to #66729 #181360 #199569.
Recently, I encountered an issue when trying to add language-specific token colors using editor.semanticTokenColorCustomizations. As noted in VS Code's semantic highlighting guide, the current implementation only supports grouping tokens under a single language.

This patch extends support for multiple languages by allowing definitions like class:c,cpp,python, enabling semantic token customization across multiple languages.

[Colibrow]

@microsoft-github-policy-service agree

[Colibrow]

@aeschli Hi, Could you please take a look?

[aeschli]

Isn't it already possible to do the same by repeating the same rule?

"variable.readonly:java": "#ff0011"
"variable.readonly:javascript": "#ff0011"

[Colibrow]

Isn't it already possible to do the same by repeating the same rule?

"variable.readonly:java": "#ff0011"
"variable.readonly:javascript": "#ff0011"

Yes. I do it by this way currently. But as for a theme settings for some languages for example:

"variable.readonly:java": "#ff0011"
"variable.readonly:javascript": "#ff0011"
"variable.readonly:cpp": "#ff0011"
"variable.readonly:c": "#ff0011"

"variable.variable:java": "#ff0011"
"variable.variable:javascript": "#ff0011"
"variable.variable:cpp": "#ff0011"
"variable.variable:c": "#ff0011"
...

It looks a little redundant.