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/_meta.lua

@@ -15,7 +15,7 @@ vim.fs = require('vim.fs')
 vim.func = require('vim.func')
 vim.glob = require('vim.glob')
 vim.health = require('vim.health')
-vim.highlight = require('vim.highlight')
+vim.hl = require('vim.hl')
 vim.iter = require('vim.iter')
 vim.keymap = require('vim.keymap')
 vim.loader = require('vim.loader')