Introduce `dart doctor` & `hook/doctor.dart`

[dcharkes]

Flutter has a flutter doctor that helps installing prerequisite tools.

Use cases:

• package:native_toolchain_c reporting whether all toolchains are available. (Make dart build / dart compile exe / dart compile aot-snapshot produce code that can be profiled with native profilers #54254 (comment))
• package:native_toolchain_rust reporting whether the Rust compiler is installed.
• package:jni reporting whether JAVA is available.

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.

Each package has a hooks/doctor.dart, so that dart doctor can run all those doctor hooks for all transitive dependencies.

cc @mosuem @mit-mit @bkonyi