Update and modernize code and QA

[gmazzap]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes/features)
• [x] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature.

What is the current behavior? (You can also link to an open issue here)

• Production autoload uses a custom autoload mechanism is used, not making use of Composer-generated static autoload
• Deprecated code is uses (see #12)
• Some dev dependencies are outdated
• QA does not use reusable workflows
• Several QA errors are found by updated Psalm
• There are zero tests

What is the new behavior (if this is a feature change)?

• Composer-generated static autoload is used for producion autoload
• Deprecated code is removed (fix #12)
• Add dev dependencies are updated
• QA uses reusable workflows, excluding the unit tests workflow which needs customization to test different Composer versions
• All QA errors are fixed
• Unit tests are set up, and a couple of tests for edited code are added

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

Other information:

Added .gitattributes.