Rename language.Scala2

[odersky]

The Scala2 language option is misleading. On several occasions I met people who had the impression that it would be a mode where Scala 2 is fully supported but none of the Scala 3 features would be accepted. This is not what it is, of course. So I propose to rename to
s2compat. I.e.

 -language:s2compat

as a command line option and

import language.s2compat

as an import.

[OlivierBlanvillain]

I agree that the current name is misleading but I think I would prefer scala2compat over s2compat.

Also I have a feeling that we should put this under a dedicated flag instead of it being a language feature. For instance, its quite surprising that -language:_ enables the compatibility mode.

[odersky]

I believe anybody using -language:_ gets what they asked for :-)

[smarter]

We should just remove support for -language:_, it doesn't mean anything useful. But even then, I think it's strange that some -language flags just turn off warnings whereas others change the semantics of the language (and are therefore much more dangerous), I think we should clearly separate these two sets of options, in particular turning off language warnings should look like turning off any other kind of warning (something like the -Wno-xxx options in clang/gcc ?)

[som-snytt]

My newly teenaged child advises me that -language:_ means "leave me alone!" Also, if you put underscore on both ends, you get leave me alone! That is, don't hassle me about features, or in pipe organ lingo, "pull out all the stops."