CI: Adding basic checks in a new workflow

[Saransh-cpp]

The application right now lacks tests, we can add a couple of basic ones -

• Lint tests (would make sure that no one is using a deprecated API or an older version of Flutter)
• Making sure the apk builds (would make sure that a PR is not destroying the app completely)

These tests will run on push and on a PR which might make the reviewing process a bit easier. I can take this up if we plan on adding this!

[yashlamba]

@Saransh-cpp Sounds good! Along with the apk check, you can also add flutter web build check as well (that's what we primarily use)

I'll rename tests -> checks so that we don't confuse them with actual flutter tests later.

[Saransh-cpp]

Great!