Implement RFC, rename tag "class" to "extern" locked behind a flag, tenary operator

[karl-police]

Implementation for luau-lang/rfcs#131 incase it gets merged

It's like #1926, but different

[aatxe]

This is the correct way to do it, not the other way, but it needs a rebase. The flag should not be a debug flag.

[karl-police]

Hmm, without Debug at the front. I wonder if this will error out Unit Tests that use :is("class"), most likely yes

[karl-police]

I guess there are no Unit Tests.

[aatxe]

you should flag those tests to change to using extern when the flag is enabled

[karl-police]

you should flag those tests to change to using extern when the flag is enabled

the new one?

[aatxe]

not if it's new, but if there were any existing tests, you'd flag them to reflect the change

[karl-police]

not if it's new, but if there were any existing tests, you'd flag them to reflect the change

It doesn't look like there were, and if there were it would have failed the build checks. But for future changes, I will look out for that. (Well, if the build fails I have to fix it regardless)