Ensure Dart version is at least as high as dependencies

[Levi-Lesches]

Say I have the following project:

environment:
  sdk: ^3.0.0

dependencies:
  pubspec_parse: ^1.5.0

This compiles, Pub doesn't complain, and this tool validates it with no issues. However, pubspec_parse version 1.5 requires Dart 3.6! Everything works because, coincidentally, I happen to have Dart 3.6 on my PC. But someone else might not, especially a CI workflow (this just happened in #138 but it happens to me all the time). I have a use case where I need to be entirely offline, so I'd like to know if I need to upgrade my Dart SDK as far in advance as possible.

To handle this, the tool should notice that pubspec_parse requires Dart 3.6 and flag that my environment.sdk is less than that. To handle the transitive case as well, the tool can look through .dart_tool/package_config.json and use the highest languageVersion it finds.

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I filed dart-lang/pub#4488 to see if the Dart team wants to integrate this into Pub directly as well