Changes 6: Version class improvements and unit tests

[bastianallgeier]

This PR …

• 🚨 Merge first: Changes 1: Refactor content storage to use new VersionId class #6436
• 🚨 Merge first: Changes 2: Plain Text Handler Refactoring #6439
• 🚨 Merge first: Changes 3: New Version class #6442
• 🚨 Merge first: Changes 4: Refactor PlainTextContentStorageHandler to use Language object #6448
• 🚨 Merge first: Changes 5: ContentStorageHandler Abstract #6449

Refactoring

• Move more logic from ContentStorage to Version where it makes sense
  • Version::contentFile
  • Modified Version::delete to always delete all languages
  • Version::touchLanguage
• Pass Language objects to storage methods
• Return null in ::modified if a version does not exist.
• Add unit tests

Outlook

• The new VersionTest::setUpSingleLanguage and VersionTest::setUpMultiLanguage methods turn out to be super useful. Changes 7: Move ContentStorage methods to ContentStorageHandler #6454 will implement a new Kirby\Content\TestCase class with those setup methods and the PlainTextContentStorageHandlerTest and VersionTest classes will both extend them, which makes the tests a lot easier. We will also be able to use this for the MemoryContentStorageHandlerTest class later Changes 10: New MemoryContentStorageHandler #6457

Breaking changes

None

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and checks all pass

For review team

• Add changes & docs to release notes draft in Notion (not relevant as the Version class is added in v5 alpha 1)

[bastianallgeier]

@lukasbestle I think we can ignore the coverage. Codecov is considering the Language::single line uncovered although it clearly is covered by multiple tests.

[lukasbestle]

It was covered, but not marked as covered 😉