* Introduce a git submodule to pin the version of flutter SDK under
  `vendor/flutter`.
* Use direnv to add `vendor/flutter/bin` to the current PATH so that
  subsequent calls to `flutter` on the shell uses the our pinned
  version of flutter.
* Update instructions to use pinned version of flutter.
* Pin to 18340ea16c of flutter which matches the current lowerbound
version in pubsec.yaml (3.21.0-12.0.pre.26).

NOTE: Users can still choose to opt-out and use their own version of
flutter. This just provides a recommended and tested version on our
CI.

Closes zulip#15

The `check-flutter-main` job serves 2 purposes:
1. It checks that the code works with the flutter's latest main
   channel.
2. It checks that our code environment setup works with a typical
   Flutter SDK installation setup (via ~/flutter).

Co-authored-by: Greg Price <[email protected]>

Co-authored-by: Greg Price <[email protected]>

Co-authored-by: Greg Price <[email protected]>

Co-authored-by: Greg Price <[email protected]>