Enable semanticTokens by default

[ericdallo]

I'm creating this issue to discuss what is needed for us to enable the semanticTokens feature as default.

@sebastiansturm worked hard to improve that feature and after some fixes and improvements it seems to me that it works well enough to be enabled as default, at least for Clojure and Dart languages that I use all the time and that have support for that already.

We can use this issue to track missing minor issues with that if exists any.

c/c @yyoncho

[ericdallo]

We should probably update the features tutorial explaining what is and how to disable/enable semantic-tokens: https://emacs-lsp.github.io/lsp-mode/tutorials/how-to-turn-off/