refactor: rename vim.highlight => vim.hl

Problem:
- `vim.highlight` module does not follow `:help dev-name-common`, which
  documents the name for "highlight" as "hl".
- Shorter names are usually preferred.

Solution:
Rename `vim.highlight` to `vim.hl`.

This is not a breaking change until 2.0 (or maybe never).

runtime/lua/vim/diagnostic.lua

@@ -1498,13 +1498,13 @@ M.handlers.underline = {
         end
       end
 
-      vim.highlight.range(
+      vim.hl.range(
         bufnr,
         underline_ns,
         higroup,
         { diagnostic.lnum, diagnostic.col },
         { diagnostic.end_lnum, diagnostic.end_col },
-        { priority = vim.highlight.priorities.diagnostics }
+        { priority = vim.hl.priorities.diagnostics }
       )
     end
     save_extmarks(underline_ns, bufnr)