Support 3.16 semantic tokens

[yyoncho]

I noticed that the latest version of the server has semantic tokens implementation but it is not following the 3.16 spec. It will be nice to do so in order to have that functionality in other clients without the need for additional client-side code.

[0dinD]

I agree, this would be ideal. The work on the current semantic tokens implementation started before it was standardized in LSP, and that's why it's using a custom command, used in the semantic tokens provider of the VS Code extension. Implementing it via LSP instead would also provide benefits in the VS Code extension, such as being able to provide semantic tokens in lightweight mode (where custom commands are disabled).

I don't have time to work on it right now, but if no one else does I'll probably have time in a few weeks. Implementing it should just be a matter of upgrading LSP libraries (if needed), and moving the semantic token commands from JDTDelegateCommandHandler to the correct place in the LSP implementation.