refactor: organize repository as a pub workspace

[marandaneto]

💡 Motivation and Context

Organizes the repository as a Dart pub workspace to enable better multi-package management and shared dependency resolution.

What changed

• Moved the plugin package into posthog_flutter/ subdirectory (with all native code: android/, ios/, macos/, lib/, test/)
• Created a workspace root pubspec.yaml that declares posthog_flutter and example as workspace members
• Added resolution: workspace to both member pubspec.yaml files
• Bumped SDK constraint to ^3.5.0 (minimum for workspace support)
• Aligned flutter_lints version across workspace (^5.0.0)

CI / Release updates

• ci.yml: Simplified to single flutter pub get at root (workspace resolves all); test runs from ./posthog_flutter
• publish-pub-dev.yml: Version validation reads from posthog_flutter/pubspec.yaml; flutter pub publish runs from ./posthog_flutter; changelog read from posthog_flutter/CHANGELOG.md
• codeql.yml: Simplified dependency install to single flutter pub get
• publish.yml: No changes needed (uses setup action + scripts)
• setup/action.yml: No changes needed (single flutter pub get resolves workspace)

Other updates

• scripts/bump-version.sh: All paths prefixed with posthog_flutter/
• Makefile: Paths updated for new structure
• RELEASING.md: Manual publish instructions updated

💚 How did you test it?

• flutter pub get resolves successfully at workspace root
• All 93 tests pass (flutter test from posthog_flutter/)
• dart analyze passes (only pre-existing info-level warnings in example)

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.