chore: add pull request template

[MAUstaoglu]

Adds .github/pull_request_template.md. The repo had no template and no CONTRIBUTING.md.

Why

The two things easiest to miss in this monorepo are per-package version bumps and CHANGELOG entries. Every package here is published to pub.dev independently, so a user-visible fix that doesn't bump pubspec.yaml never actually reaches anyone. #4 is a live example — a correct, well-tested fix that would have shipped to nobody.

The template also:

• Asks whether the change was verified on real hardware, not just the simulator. The simulator sandbox is more permissive and it renders on the host Mac GPU, so it masks device-only failures. Both sqflite_tvos: resolve getDatabasesPath under Library/Caches (Documents isn't writable on device) #4 (Documents not writable on device) and the recent tvOS <26 metallib crash were invisible in the simulator.
• Prompts contributors to flag behaviour changes — a changed default path or return value that silently alters what an existing app sees deserves its own changelog line, not just a "fix" bullet.
• Asks about sibling packages affected by the same tvOS constraint, so a platform-wide finding doesn't get fixed in one package and forgotten in five others.
• Reminds maintainers in a footer that pub.dev publishing is irreversible and a version number can never be reused.

Modelled on the flutter-tvos template, adapted for per-package versioning (no dev branch here, and no shared [Unreleased] section since versions are independent).