✨ Adds workspace support

[AlexV525]

What does this change?

Add pub workspace support to FlutterGen in both execution modes.

This PR:

• adds build_runner --workspace support to flutter_gen_runner
• adds fluttergen --workspace support to the command package
• preserves package-local pubspec.yaml and build.yaml behavior for each workspace member
• adds a real workspace example plus CI coverage for the new flows
• documents the required minimum versions: Dart >=3.7.0 and build_runner >=2.12.0

Resolves #731 🎯

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.37%. Comparing base (f5ce644) to head (b308cc3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #739      +/-   ##
==========================================
+ Coverage   96.69%   97.37%   +0.67%     
==========================================
  Files          24       24              
  Lines         969      991      +22     
==========================================
+ Hits          937      965      +28     
+ Misses         32       26       -6     

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