- Standardized the layout of `pubspec.yaml` for all templates.
  - Consistent ordering of all top-level keys
  - Use `^` notation for versions.
  - Define the minimum SDK to be `1.20.1`.
- Simplified `.gitginore` for all templates.
  - No longer need to ignore `packages` directory. They are not not generated
    with SDK 1.20.1+.
  - No longer ignoring compiled outputs by extension. These will all be in the
    `build` directory, which is already ignored.
  - No longer ignoring JetBrains IDE files. These should be ignored in a users
    global `.gitignore`.

Also added tests to validate many of these things

Fixes https://github.com/google/stagehand/issues/372