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/lsp/handlers.lua

@@ -437,7 +437,7 @@ function M.signature_help(_, result, ctx, config)
   local fbuf, fwin = util.open_floating_preview(lines, 'markdown', config)
   -- Highlight the active parameter.
   if hl then
-    vim.highlight.range(
+    vim.hl.range(
       fbuf,
       sig_help_ns,
       'LspSignatureActiveParameter',