Restore semantic token flickering workaround removed in #5697

[charlespierce]

Closes #6452

Info

• As discussed in Syntax Highlighting in VS Code Flickers when typing quickly #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

• 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

• 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?

[kjeremy]

LGTM!

[kjeremy]

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)