Merge #6470

6470: Restore semantic token flickering workaround removed in #5697 r=kjeremy a=charlespierce

Closes #6452 

Info
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* As discussed in #6452, the `Error('busy')` workaround for semantic token flickering was removed because the underlying issue was believed to be fixed in VS Code.
* It turns out that the fix isn't yet complete, so this caused flickering of the semantic highlighting when making rapid edits (e.g. typing quickly).
* This PR restores that workaround and makes it slightly more robust, covering all areas of semantic token middleware.

Changes
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* Added middleware functions for `provideDocumentSemanticTokens`, `provideDocumentSemanticTokensEdits`, and `provideDocumentRangeSemanticTokens` to match the 3 possible middleware hooks defined in https://github.com/microsoft/vscode-languageserver-node/blob/master/client/src/common/semanticTokens.ts#L33
* Each intercepts a `null` or `undefined` return and throws an error with the message `busy` instead, which prevents the tokens from being removed and re-added (causing the flickering behavior)

Tested
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* Tested locally that the flickering behavior is gone.
* There don't appear to be any significant tests of the VS Code plugin side of things, other than that it loads. Is there somewhere I can / should add tests to cover this behavior?

Co-authored-by: Charles Pierce <[email protected]>

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