Introduce dart doctor

[dcharkes]

Flutter has a flutter doctor that helps installing prerequisite tools.

We've discussed adding a similar dart doctor (#54254 (comment)) but didn't have a tracking issue yet.

One use case is C linker invocation from the Dart tooling (#49418).

However, we might have arbitrary tools being invoked by build.dart or link.dart hooks (#50565). A rust compiler, a C linker, a Go compiler, a Wasm compiler, some Java tooling, a JSON minifier, you name it.

In those cases it might make more sense that each package has a bin/doctor.dart. So that one would run dart run intl doctor.

Now, such packages would then likely shell out to helper packages again native_toolchain_c, native_toolchain_rust.

If we make dart doctor basically invoke all bin/doctor.dart of all packages in sequence that would lead to duplicate reports if multiple packages use the same helper package. (Maybe that is okay?)

If we would like to catch duplicate reports before reporting back to the user, we'd need a hook/doctor.dart in every package instead. And for that we would probably need to somehow make the tools abstraction (dart-lang/native#856) public and serializable. (Which might be a bit far fetched.)

Before committing to a dart doctor, we could prototype something in package:dart_doctor.

cc @mosuem @mit-mit @bkonyi