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/_editor.lua

@@ -32,7 +32,7 @@ for k, v in pairs({
   func = true,
   F = true,
   lsp = true,
-  highlight = true,
+  hl = true,
   diagnostic = true,
   keymap = true,
   ui = true,
@@ -1197,4 +1197,7 @@ require('vim._options')
 ---@deprecated
 vim.loop = vim.uv
 
+-- Deprecated. Remove at Nvim 2.0
+vim.highlight = vim._defer_deprecated_module('vim.highlight', 'vim.hl')
+
 return vim