Language support for Luau .luau

[Jerse1]

If you want an unsupported language added, provide:

• language name: Luau
• file extension(s): .lua, .luau
• method(s) of commenting text: The same as Lua 5.1, since it's backward-compatible.
  -- comment here
  --[[
  comment here
  ]]
• location of sample code:
  https://github.com/dphfox/Fusion
  For other enhancements:
  Give a concise description of what you want cloc to be able to do.
  https://luau-lang.org/

[AlDanial]

Can't use .lua as an extension for Luau since it already maps to Lua (and I don't know of a way to tell whether a .lua file contains Lua or Luau).