Support for FFI calls with `@Native external` functions through Native assets on Windows. This enables bundling native code without any build-system boilerplate code.

For more info see:

* https://github.com/flutter/flutter/issues/129757

### Implementation details for Windows.

Mainly follows the design of https://github.com/flutter/flutter/pull/134031.

Specifically for Windows in this PR is the logic for finding the compiler `cl.exe` and environment variables that contain the paths to the Windows headers `vcvars.bat` based on `vswhere.exe`.