[TASK] Update Ext to v13 compatibility

This MR adjusts the Extension for TYPO3 13 LTS compatibility.

In addition for a better maintenance, runTests.sh is added and basic code analysing is checked for merge requests and main. The guides for code styling and analysing is adopted to TYPO3 core code styles with two differences:

• Code Style fixer checks for strict type declaration
• phpstan is up to level 8 instead of core used level 5

For continuing work with bugs and other issues, the TYPO3 Testing Framework is added for running functional and unit tests.

As I didn't see any chance for unit testing simple parts, I added only a dummy test. This can be removed after a regular unit test is added.

On functional test side, this MR now checks the following (barely minimum) parts:

• FormToDatabaseFinisher writes the data to the database
• FormResultsController exports the CSV
• DeleteFormResultsCommand deletes results older than the default 90 days

As long as there could be more testing, but for the start this should help improving code quality and testing coverage at all.

Fixes #114 (closed) Fixes #118 (closed)