Combine SearchableComboBox and TextField for adding fields in Custom Entry Types

[Memeel]

Closes #14547

This pull request replaces the separate mechanisms (SearchableComboBox and free TextField) for adding new fields with a single TextField utilizing auto-completion in the "Custom Entry Types" preference pane.

This improves usability by unifying the workflow: users can quickly filter and find a standard field or type a custom field name directly in the same control. The option is located under File -> Preferences -> Entry types.

Steps to test the combined input field

1. Go to File -> Preferences -> Entry types.
2. Select any entry type from the list on the left.
3. In the "Required and optional fields" section, locate the new "Type new field..." text input at the bottom.
4. Verify the auto-completion for standard fields:
   • Start typing a standard name (e.g., day).
   • Verify that the list suggests matching fields.
   • Select a field and click the + button.
   • Verify it is added to the list.

5. Verify the creation of custom fields:
   • Type a custom name that matches no suggestions (e.g., customField).
   • Verify that the input is valid (no red border) and the + button is enabled.
   • Click + to add it.

Final Result

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[Siedlerchr]

In general I like this; however, the drawback is that you need to know the field names to search for...

[koppor]

This is not user-facing - which user knows what a "SearchableComboBox" is - this is not a word from an English dictionary. - Please rephrase.

[jabref-machine]

JUnit tests of jablib are failing. You can see which checks are failing by locating the box "Some checks were not successful" on the pull request page. To see the test output, locate "Source Code Tests / Unit tests (pull_request)" and click on it.

You can then run these tests in IntelliJ to reproduce the failing tests locally. We offer a quick test running howto in the section Final build system checks in our setup guide.