Setup initial pipeline

[davidhoelzel]

Code quality tools:

• php-cs-fixer
• phpstan
  • to be discussed as most symfony bundles do not use it either (mostly due to redundant array-shape definitions etc.)
• phpunit

For phpunit, the setup in the application is already done (version 10.5) without the symfony unit bridge.

Perhaps we should add a code-coverage badge as this will be a public bundle.