Support `---@enum`

[numToStr]

This is very useful in the actual code as we don't have to create separate ---@alias for describing enums but the problem is that we have to parse tables to get all the enum variants

Reference

• https://github.com/sumneko/lua-language-server/wiki/Annotations#enum
• [Req] —-@enum LuaLS/lua-language-server#1255 (comment)