RFC: Rename tag "class" to "extern" for ExternType

[karl-police]

Rendered

is("class")
ìs("extern")

[andyfriesen]

Sounds good to me!

[karl-police]

Sounds good to me!

I made luau-lang/luau#1926 and luau-lang/luau#1927

two variations

[karl-police]

It turns out that ìs("class") is actually not used anywhere in the Unit Tests in the first place as well. I could remove Debug infront of the flag and let it turn on through --fflags=true, and allow that all current is("class") will just error.
Unless it's worth to wait and implement a warning that points out to use "extern"

[aatxe]

It's still a feature in the beta, and not usable elsewhere, and it will never break running code, so I think it's fine for us to not try to manage an awkward "warn to migrate" period (which is, intrinsically, the same as producing the error anyway).