chore: add typed definitions for _G

[cayasde]

why
luau currently infers _G as any. this pr adds explicit typings to improve type checking and autocompletion.

supported types:

name	type
__DEV__	boolean?
__IGNORE_INSTANCES_WARNING__	boolean?
NOCOLOR	boolean?

vscode setup
if you're using vscode with luau-lsp, add the following to your .vscode/settings.json:

{
    "luau-lsp.types.definitionFiles": [
        "types/_G.d.luau"
    ]
}