fix(keepSyntax): correct regex to avoid unnecessary escaping of speci…

[LuvForAirplanes]

Before, when you would simply open a Markdown file that had Wikilinks in it, (for example: [[Test Link]] ) NextCloud would automatically escape the double brackets to look like this: [[Test]]
This caused programs like Obsidian to lose the linking capability. The regex in the keepSyntax.js file was corrected to handle characters individually.

📝 Summary

• Resolves: Escaping of square brackets breaks links of embedded attachments for external Markdown Editor #4795

🖼️ Screenshots

🏚️ Before	🏡 After

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

[LuvForAirplanes]

Not sure why my sign off was rejected.