Support semantic tokens protocol (#1721)

* [WIP] Add rudimentary semanticTokens support

* Use buffer-local advice

* Don't request semantic tokens if unsupported

* Use lsp-feature? to detect semanticTokens support

* Prevent out-of-bounds access

* Update semantic highlighting docstrings

* [untested] Add support for token modifiers

clangd returns tokenModifiers = [], so I'll need another language server
to test this against. There's a new defcustom to enable/disable modifier
highlighting that defaults to disabled for the time being

* Add some token types defined by clangd

* Mention semantic tokens support in readme

* Remove leftover `append'

* Remove unused variables

* Fix several issues raised by @yyoncho

Author: sebastiansturm

README.md

@@ -64,9 +64,7 @@ most popular Emacs packages like `company`, `flycheck` and `projectile`.
   - Ivy integration - [lsp-ivy](https://github.com/emacs-lsp/lsp-ivy/)
   - Treemacs integration -
     [lsp-treemacs](https://github.com/emacs-lsp/lsp-treemacs)
-  - Semantic highlighting (as currently implemented by JDT LS and
-    unreleased builds of clangd, cf. [Semantic highlighting
-    spec](https://github.com/microsoft/vscode-languageserver-node/pull/367))
+  - Semantic highlighting (using either the theia semantic highlighting protocol as implemented by JDT LS and some versions of clangd, or the semantic tokens protocol defined by LSP 3.16)
   - [which-key](https://github.com/justbur/emacs-which-key/) integration
     for better discovery