feat: Add Swift Package Manager support

[spydon]

Description

Adds Swift Package Manager support to gamepads_ios and gamepads_darwin, following the official Flutter plugin migration guide. This removes the hint emitted by flutter pub get on Flutter 3.44+:

The following plugins do not support Swift Package Manager for ios:
  - gamepads_ios
The following plugins do not support Swift Package Manager for macos:
  - gamepads_darwin

Changes per plugin:

• Moved the Swift sources from Classes/ into the SPM layout (ios/gamepads_ios/Sources/gamepads_ios/ and macos/gamepads_darwin/Sources/gamepads_darwin/).
• Added a Package.swift manifest with the GameController framework linked explicitly.
• Updated the podspec source_files to the new path, so CocoaPods-based apps keep working unchanged.
• Bumped podspec platform minimums to iOS 13.0 / macOS 10.15 to match the Flutter 3.35+ constraint already in the pubspecs (the old 11.0 / 10.11 values were below what Flutter supports).
• Added .build/ and .swiftpm/ to the root .gitignore.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #108