Define a vscode.executeSemanticHighlightProvider command

[aeschli]

Providers also define a command so that extensions can use them. See
https://code.visualstudio.com/api/references/commands
for examples.

The semantic highlight provider should offer something similar.

Uses cases:

• get semantic highlighting for a virtual document

[mjbvz]

What we want for the markdown preview is actually #56356

[alexdima]

@aeschli I don't know exactly what to do here. Should this block in any way the stabilization of the API? The SemanticTokens are not sufficient without a legend to help decipher the result. i.e. it is not like a request to provide references, etc. which work without a leged to decipher the result.

[aeschli]

Hm, no one has really asked for that feature yet, so it's hard to justify this a to be a blocker of any kind.

But yes, it seems that the new API is not suited to be used programmatically.

Let's say the legend comes as part of the result. But that means when using virtual documents to get semantic tokens for the pieces of embedded languages, you might receive tokens types/modifiers back that you didn't anticipate. In order to use them you would have to re-register the provider with a new legend, or filter these.