Refine parsing format of CSL reference marks

[subhramit]

Follow-up to: #11712

• Use more strict regex to exactly match the format, disallow multiple whitespaces, disallow absence of unique ID, etc.
• A lot of code I wrote for CSL4LibreOffice - E [Add re-distribution of numeric CSL citations] #11712 may seem like black-magic, as the fix was needed as soon as possible. I will slowly be documenting the code to aid this. Some JavaDoc is added in this PR as well.

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.