Use mono_repo for package:web (#288)

Splits package:web into two packages:
- package:web
- package:web_generator (not published)

The latter contains our tooling to generate the former, and used to be the tool directory.

- Refactors code to point to new paths (and using `Platform.script.resolve` so that the `bin/` files can be run from anywhere instead of within that directory)
- Moves directories like `third_party` and tests to their new appropriate locations
- Moves CI configurations to `mono_repo.yaml` and `mono_pkg.yaml` files and is now generated as `dart.yml` (used to be `build.yaml`)
- Updates READMEs to reflect the package split
- Adds new LICENSE, `pubspec.yaml`, and CHANGELOG files
- Removes `JSArrayExtension` which is no longer needed.

Refactors code to point to new paths and moves directories like third_party to their appropriate locations. Moves the CI configurations to mono_repo.yaml and mono_pkg.yaml files, and is now generated as dart.yml.