Add warn

[gaymeowing]

Lune currently has a warn global just like Roblox, its odd Lute doesn't currently.
Whilst warn shouldn't be used instead of ANSI color strings generally, its nice to use whilst print debugging to have prints with a different output color easily. As print debugging (at least for me) is usually quick and dirty, so I don't really want to have to import in a ansi color module or copy and paste a warn function

[aatxe]

What does warn do? Print to stderr instead of stdout?

[gaymeowing]

What does warn do? Print to stderr instead of stdout?

Prints to stdout, but has yellow ansi coloring applied, and if color is unable to be used it'll print an extra line above that says [WARN]

[aatxe]

That sounds surprising to me, personally. Why should the runtime provide a function to print normally, but in yellow? You can write that yourself, you can even name it warn today.