Make TASTy files of stdlib distinguishable

[prolativ]

As discussed here https://github.com/lampepfl/dotty/pull/14156/files#r786863638
we need a proper way to be able to tell if a TASTy file comes from the standard library or not as this is fact is important for the procedure of checking references during forward compatible compilation.

I assume this would require extending TASTy format by adding some kind of a metadata section. The question is probably if there is any additional information that might be worth adding to this section and if it should be designed in a more generic way so that arbitrary metadata could be added there without changing TASTy format later on.

[bishabosha]

a cheap but nasty solution could be to add some extra text in the "toolingVersion" field of the tasty header, that is recognised by the forward compat analysis