Implement _G and _ENV

`_G` is a global variable that simply refers to the global environment (it can be set by the user to something else but it does not change any behavior).

`_ENV` is used when binding/referencing *free names*. The user can change `_ENV` and it will affect where global variables are stored and how names are looked up.

See: https://www.lua.org/manual/5.3/manual.html#2.2