fix: Exclude Readme.md files from file locking in the text app

[rikled]

📝 Summary

• Resolves: README.md is almost always locked

This PR excludes Readme files from file locking. Without it is almost impossible to delete these files as they are opened as soon as a user visits the directory of the file. This is meant as an intermediate solution until #5597 is implemented.

🏁 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

[silverkszlo]

I've tested this by adding temporary debug logging and saw the following output in the nextcloud log, so seems to work :)

{"message":"Attempting to lock file","data":{"fileId":"89","fileName":"regular.md"}}
{"message":"Lock result","data":{"fileId":"89","fileName":"regular.md","isLocked":"true"}}

{"message":"Skipping lock for file","data":{"fileId":"86","fileName":"Readme.md","reason":"Readme.md exception"}}