Drop PHP 5 support

[gocom]

We should increase required PHP version to 7.2, dropping unsupported PHP versions in PHP-Textile 4.0.0.

This should be done with minimal code changes to make transition smoother, but we should add strict type hints, return types and interfaces for both the parser and its configuration getters/setters.

This would make the library much more useable in modern context, prepare it for future improvements and being able to use newer PHP features, and resolve issues with dev/test dependency blockers. Itself the usage of the library and API would stay the same.

Process:

• Change the current (private, but) protected properties and methods to private.
• Add strict-type checks, and argument and return types.
• ParserInterface and ConfigInterface, both implemented by the current parser.
• Update development dependencies.
• Remove getVersion() and version number from Parser to simplify release process.

Further:

• After getVersion is gone, automate release process with GitHub actions.
• After automation and version bumps are unnecessary, change branching model to PR only. Disallow direct pushes to master; feature branches only. Tests must pass before branch can be merged. Non-maintainer PRs require approvals.