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

runtime/lua/vim/highlight.lua

@@ -5,6 +5,7 @@ local M = {}
 M.priorities = {
   syntax = 50,
   treesitter = 100,
+  semantic_tokens = 125,
   diagnostics = 150,
   user = 200,
 }