feat: fvm support

[pedroafmonteiro]

Closes #296

This pull request extends Flutter version detection to support FVM config files in addition to pubspec.yaml. I decided to try to implement this feature due to changes on NIAEFEUP/uni #1814. Actions are failing because it cannot detect this file.

• updated the documentation in README.md to explain that Flutter version can now be sourced from .fvmrc or fvm_config.json, in addition to pubspec.yaml.
• modified the flutter-version-file input description in action.yaml.
• the setup.sh script now detects FVM config files and extracts the Flutter version using jq, while continuing to use yq for pubspec.yaml. channel/version assignment logic for special cases like "stable", "beta", "master", and "main" continue to be supported.
• windows runner logic in action.yaml only installs yq if the version file is not an FVM config file, since FVM config files are parsed with jq instead

[coderabbitai]

removed by @bartekpacia

[coderabbitai]

REMOVED by @bartekpacia

[pedroafmonteiro]

Hey @bartekpacia, any chance you could check this out? This would help very much the development of many projects here. Thanks! :)

[bartekpacia]

Ooo hi @pedroafmonteiro! Sure let me take a look:)

[kuhnroyal]

Linking some issues here for reference: #101 #296

The topic of FVM support has been discussed multiple times in the past and was decided against for different reasons. Personally, I would be glad if I could sunset https://github.com/kuhnroyal/flutter-fvm-config-action

[bartekpacia]

Thanks @kuhnroyal for sharing the additional context.

@pedroafmonteiro could you edit the PR description to include "resolves #296"?

Oh, and please also rebase with master so that CI will pass. It was caused by #387 which I have just fixed.

[pedroafmonteiro]

Hi! Yes, I will fix those issues right now, sorry for the delay!

[bartekpacia]

Thanks!