Add ValueOf equivalent of SIP23 (TLS) to dotty

[soronpo]

SIP23 will be merged into Scala 2.xx sometime in the future (currently merged into TLS).
Its implementation has a ValueOf type class that enables us to request the literal values from literal types. E.g.:

def foo[T](implicit i : ValueOf[T]) : T = valueOf[T]

Since we want to have Scala 2.xx and Scala 3.xx to be the same in the future, it is important to keep this feature.

[propensive]

I would find this potentially very useful.

[smarter]

Duplicate of #2616 (I'll fix it soon!)