Ensure Dart version is at least as high as dependencies

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.

I filed https://github.com/dart-lang/pub/issues/4488 to see if the Dart team wants to integrate this into Pub directly as well

Start by locating the validator's existing SDK and dependency checks, then inspect .dart_tool/package_config.json to understand the available languageVersion data. Implement validation that compares the project's environment.sdk with the highest required Dart version, including transitive packages, and verify that incompatible constraints are reported.