Support explicit language version declaration in pubspec.yaml

[jonasfj]

[parlough]

Is this still desired? I think the SDK constraint lower-bound being the language version is well established now and makes sense.

[sigurdm]

I think we still dream of this.

Having to write

environment:
  sdk: ^3.5.0

Is a bit of a mouthful, and not all that intuitive. If all that we want to express is something along the lines of

dart: 3.5

There are several reasons we haven't done this yet. At least:

• It is not clear what happens if you have conflicting sdk constraint and language version (can you require a newer sdk, but still be written in an older language? do we want to allow that?)
• 3.5 unquoted is a float not a string in yaml, complicating the parsing quite a bit.