Set up CI: test, analyze, and generate

[gnprice]

We should run our tests and other checks in a CI setup.

These consist of:

• flutter test
• flutter analyze
• Regenerating all generated files, and checking that nothing changed. This means:
  • The build_runner command mentioned in the README (but can use build instead of watch).
  • After Store some user data locally #13, a couple of drift_dev commands that need to be run whenever the database schema changes; they'll be described in a comment at the spot where we define the current schema version.

(If we complete this before #13, that's fine; we'll leave out that last item, and just add it as part of #13.)

Some things that we'd ideally have but are optional for this issue (and we'll just file followup issues if they're not in the first implementation of this one):

• A handy wrapper script for local development use, like tools/test in zulip-mobile, that makes all the same checks we do in CI.
• That wrapper script being (again like tools/test in zulip-mobile) efficient about not redoing checks for things that shouldn't be affected by the local changes.
• That wrapper script being written in Dart, rather than Bash.

[gnprice]

Another form of generated file we'll want to include:

• the icon font and corresponding IconData code, from Build an asset font for custom icons #200