General project improvements

[pablojimpas]

• I discovered this Flutter app recently and when I cloned it my IDE show a bunch of warnings that I've addressed in these PR.
• I've also updated all packages that were resolvable to new major version, the only breaking change introduced that I had to fix was with the class FormBuilderValidators that was moved from the package flutter_form_builder to the new one form_builder_validators.
• Some files weren't properly formatted so I ran flutter format ./lib.
• I wanted to run it in my Linux machine since I didn't have a phone at hand so I run flutter create . to add the new analysis_options.yaml and add support for Web and Linux, I've done almost no testing but it seems to work just fine (although some packages like geolocator don't support Linux)
• I've added a GitHub Action that checks for Dart warnings and runs tests on every PR or push to the repo.

Sorry for sending a PR with so many unrelated things, I should probably have partitioned this changes into different branches and commits but I just did a quick rundown through the system and thought that since the project doesn't receive a lot of PRs it will be easier to review it.

Hope you find something useful in my changes. Feel free to ask for anything.

[IoTThinks]

Glad to have some improvements. I tested the app. Still need improvements here and there in the App GUI.

However, I'm not expert in Flutter yet. But I can contribute my testing effort. :D

[pablojimpas]

I tested the app. Still need improvements here and there in the App GUI.

I've not touched the GUI in this PR, but thanks for testing this branch to make sure that these under the hood changes didn't break anything.