Update LETTER fragment to resolve Windows parsing issue

[priyanshu16095]

Refers #12364 (comment)

This PR enhances the part of the Ltwa.g4 grammar file which causes parsing issues on some Windows systems, making it compatible with Windows and ensuring proper interpretation of characters.

Steps to test

• Fetch the latest changes.
• Restart the JabRef, it will work.

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• 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.

[Siedlerchr]

can you add a comment for the other unicode chars as well like you did?
you can find out about them at https://www.compart.com/de/unicode/U+0100

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨

[koppor]

This surely was introduced by an ANTLR update. Did anyone find an issue at ANTLR or a changelog entry of them?

[calixtus]

Task :jablib:generateGrammarSource
warning(180): org\jabref\logic\journals\ltwa\Ltwa.g4::: chars 'A' used multiple times in set 'A'..'Z' | 'a'..'z'
warning(180): org\jabref\logic\journals\ltwa\Ltwa.g4::: chars 'a' used multiple times in set 'A'..'Z' | 'a'..'z'
warning(180): org\jabref\logic\journals\ltwa\Ltwa.g4::: chars '\u00E0' used multiple times in set 'A'..'Z' | 'a'..'z' | '\u00C0'..'\u00D6' | '\u00E0'..'\u00F6'