Implement SIP 57 - add runtimeChecked method and annotation

[bishabosha]

ref: https://docs.scala-lang.org/sips/replace-nonsensical-unchecked-annotation.html

Also this doesn't change synthetic code that is generated to use @unchecked, or make @unchecked meaningless for this use case - this should probably come at a later stage after the definition is no longer experimental

fixes #21012

[bishabosha]

For the future: once the definition is stabilised, we can deprecate @unchecked in this use case, switch over code generation, semantic checks, error messages to use .runtimeChecked instead of : @unchecked

And also later possibly a language mode could introduce that refutable pattern bindings become an error?

[bishabosha]

@Gedochao how can I get this reviewed?

[odersky]

I left some suggestions for the doc page. Otherwise LGTM.