Introduce GitHub workflows

[marcossevilla]

We should add GitHub workflows to improve package maintainability and enforce rules around contributing to the repository.

Currently, this CI feature is a work in progress since we're adding workflows to enforce basic Dart/Flutter related checks like linting and formatting, which will improve the code introduced in new PRs and also introduce a new test coverage threshold for each package so we can make sure the features work correctly and simplify future enhancements or refactors.