Add test cases for "UnicodeToLatex" and "LatexToUnicode"

[AbdAlRahmanGad]

I added the test case mentioned in #5547 and other test cases to ensure reliability.

The problem with the test case was that ı̄ is not one character it's a combination of ı + ̄ unlike ā. What was happennig is that ı was being converted to \i and then ̄ was being converted to {\={}} so the result would be {\i{\={}}}.

I made it that we deal with such characters that cause conflict after combining accents. They are stored in a new variable called UNICODE_LATEX_CONVERSION_MAP_AFTER_COMBINING_ACCENTS I think this will be the base to deal with such cases.

I also added some of the underdot characters as the previous implementation didn't handle them.

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.

[calixtus]

Maybe this could help: https://www.unicode.org/faq/char_combmark.html

[koppor]

I think, all the unicode should be normalized before conversion. Use the formatter introduced at #11056.

[koppor]

I made it that we deal with such characters that cause conflict after combining accents.

My comment above in other words: We need to rely on the normal form NFC. Base our internal maps on that. And do not introduce some other maps.

[koppor]

Thank you for the quick action taken. Just minor comments.

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Future work: The part at " // Combining accents" will never be touched? One should check with "code coverage"