feat(lsp): initial support for semantic token highlighting

* credit to @smolck and @theHamsta for their contributions in laying the
  groundwork for this feature and for their work on some of the helper
  utility functions and tests

scripts/gen_vimdoc.py

@@ -183,6 +183,7 @@ CONFIG = {
             'diagnostic.lua',
             'codelens.lua',
             'tagfunc.lua',
+            'semantic_tokens.lua',
             'handlers.lua',
             'util.lua',
             'log.lua',