Support package_config 3.0.0

[Carapacik]

What does this change?

Updates the package_config dependency constraint in flutter_gen_runner from ^2.1.0 to >=2.2.0 <4.0.0.

Previously, projects using package_config 3.0.0 could not use flutter_gen_runner 5.15.0 because its dependency constraint only allowed package_config 2.x:

Because flutter_gen_runner 5.15.0 depends on package_config ^2.1.0 and the project depends on package_config 3.0.0, version solving failed.

The updated constraint resolves this dependency conflict while preserving compatibility with package_config 2.2.x.

No new dependencies are introduced.

Fixes #783 🎯

Type of change

• Bug fix (non-breaking change which fixes an issue)
• 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

Checklist:

• Make sure to open a GitHub issue as a bug/feature request before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
  • Ensure the tests (melos run test)
  • Ensure the analyzer and formatter pass (melos run format)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.37%. Comparing base (7c7200d) to head (abad4fe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #784   +/-   ##
=======================================
  Coverage   97.37%   97.37%           
=======================================
  Files          24       24           
  Lines         989      989           
=======================================
  Hits          963      963           
  Misses         26       26           

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