feat: add dart_define input to flutter_package workflow

[unicoderbot]

Description

Adds a new dart_define input to the flutter_package workflow that allows passing --dart-define key-value pairs to Flutter tests. This enables testing code paths that depend on compile-time constants, such as kReleaseMode (dart.vm.product=true) or custom flavors.

Changes

• Added dart_define workflow input accepting a space-separated list of KEY=VALUE pairs
• Updated the Run Tests step to iterate over the provided defines and forward each one as a --dart-define flag to very_good test
• Added documentation for the new dart_define input with usage examples

Usage

jobs:
  build:
    uses: VeryGoodOpenSource/very_good_workflows/.github/workflows/flutter_package.yml@v1
    with:
      dart_define: 'dart.vm.product=true FLAVOR=staging'

Closes #91