Implement textDocument/semanticTokens (semantic highlighting)

[radeksimko]

I have only implemented the first handler and the protocol allows omitting the rest via capability negotiation, so we can add the other ones later.

Client always sends textDocument/semanticTokens/full almost immediately after didOpen and didChange, which means that we may not have all the schema at that point. workspace/semanticTokens/refresh would enable us to refresh tokens dynamically from the server side as we know more schema, e.g. when terraform version and/or terraform providers schema -json finish. As a result not supporting this call initially may provide occasionally "flickering UX" (colours change as the user types), but the upside is that it can't be any worse than it is, because we are just enriching the highlighting and the static grammar in VSCode extension still provides the base.

Testing requires hashicorp/vscode-terraform#523 on the client side.

Closes #264

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Follow-up issues:

• Block responding to textDocument/semanticTokens until schema is available Block responding to textDocument/semanticTokens until schema is available #343
• Support workspace/semanticTokens/refresh Support workspace/semanticTokens/refresh #344
• Support textDocument/semanticTokens/range Provide semantic tokens for document range #345
• Support textDocument/semanticTokens/full/delta Provide semantic tokens for document delta #346

[ghost]

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