Add initial checker framework configuration

[koppor]

To check @NonNull, there is the checker-framework

There is a gradle plugin.

When using, I get

java.lang.Error: Backtrace:
        at org.checkerframework.javacutil.AnnotationBuilder.fromClass(AnnotationBuilder.java:188)
        at org.checkerframework.javacutil.AnnotationBuilder.fromClass(AnnotationBuilder.java:156)
...
error: AnnotationBuilder: fromClass can't load class org.checkerframework.dataflow.qual.Deterministic
  Is the class in checker-qual.jar?
...
> Task :compileJava FAILED

FAILURE: Build failed with an exception.

• Risk: JDK 23: This plugin will not work on JDK 23 kelloggm/checkerframework-gradle-plugin#272

---

• Blocked by Add support for EISOP fork kelloggm/checkerframework-gradle-plugin#275
• Fix null issues

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• 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.