Minor code quality work

[koppor]

This ports non-http-server related changes from #9748 to a smaller PR

• Documentation / code style
  • Refined BibEntry class (JavaDoc, builder)
  • Comments to InternalField
  • move StyleTester to "test" module - package ...testutils/interactive...
• Made class "JabRefItemDataProvider" more visible
• Encoding of a .bib file can now be asked for externally
• Fixes typo in NetworkTabViewModel

Compulsory checks

Beta Give feedback

1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

org.opentest4j.AssertionFailedError: The following classes are not allowed to depend on org.jabref.preferences.JabRefPreferences ==> expected: <[]> but was: <[src/test/java/org/jabref/testutils/interactive/styletester/StyleTesterMain.java]>

[Siedlerchr]

Because you moved it to a folder under test, the architecture test says it's a violation because you depend on prefs. I tried adding an exception but does not work

[Siedlerchr]

Still failing unit architecture test

[koppor]

In d8f5b06 (#9759), I completely rewrite the checks for the test archiecture.

1. Use ArchUnit instead of self-crafted testing
2. Introduce new tests for test class naming consistency

Because of the second test, many test classes have been renamed to end with Test. We still have around ten exceptions, which are all helper classes.