Support for PHP 7.4

[simbig]

Problem

The phpstan-baseline-per-identifier library currently only supports PHP 8.0. Unfortunately, one of my current projects is limited to using PHP 7.4. This prevents me from utilizing the valuable features of this library.

Suggestion

To enable the use of PHP ^8 features while maintaining backward compatibility with PHP 7.4, I'd propose implementing a strategy similar to the one outlined in this blog post: https://tomasvotruba.com/blog/how-to-release-php-81-and-72-package-in-the-same-repository/ and done in this repo: https://github.com/TomasVotruba/cognitive-complexity/blob/main/.github/workflows/downgraded_release.yaml

This approach involves:

• Maintaining a Single Codebase: Continue developing in a single repository.
• Downgrading for PHP 7.4 Releases: Implement a build process to downgrade the codebase and dependencies to PHP 7.4 for specific releases.

If the project maintainers are interested in exploring this option, I would be happy to contribute a pull request to implement it.

[janedbal]

For such a small lib I think it is easier to just downgrade the syntax used.

If you submit a PR, I'll accept it.

[janedbal]

Released as 2.1.0, thank you!