Make it possible to set a default external text editor

### Please agree to the following

- [x] I have searched [existing issues](https://github.com/cryptomator/android/issues?q=) for duplicates
- [x] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/android/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Make it possible to set a default external text editor

### Motivation

Due to issues with the built-in text editor (#555 #588), I have started using an external one most of the time. But having to choose open with from the pull-down menu (which I do many times a day) is pretty annoying. And so it would be great if a default text editor could be configured in the settings.
One should always be able to revert to the built-in editor as default.

### Considered Alternatives

_No response_

### Anything else?

_No response_