Setting to strip trailing whitespace on save

### ⚠️ This issue respects the following points: ⚠️

- [x] This is a **enhancement/feature request**. Not a [bug or question](https://github.com/gsantner/markor/issues/new/choose).
- [x] The topic is **not** already reported at [Issues](https://github.com/gsantner/markor/issues?q=#js-issues-search). _(I've searched it and checked most recent issues)_.
- [x] Markor **is** up to date. See [Releases](https://github.com/gsantner/markor/tags) for the latest version. Updates are available from [F-Droid](https://f-droid.org/en/packages/net.gsantner.markor/) and GitHub.
- [x] The wanted feature/enhancement is not present in the latest development version (git master). (Please [download](https://nightly.link/gsantner/markor/workflows/build-android-project/master) and try the test version of Markor, named **Marder**. Don't worry; Markor and Marder appear as completely separate applications. You can install both side-by-side, and Markor's settings are not touched. If your desired feature is present, you don't need to open this issue. The change will be part of the next Markor update.)

### Description

I'd like for Markor to include a setting that removes whitespace from the end of each line when you save a file.

This would apply to every text format (that I can think of), so I think having the setting under Edit Mode would make sense.

This relates to the setting "Markdown > Syntax Highlighting > Highlight line ending". Unlike it, though, this would automate the cleanup of trailing spaces, including a single trailing space. (I'm not entirely sure why "Highlight line ending" only highlights trailing spaces when there 2 or more in the first place.)

### Information

Android version: 17
Device (phone model): Pixel 7a
App Version: 2.16.1

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log

```shell
-
```

No files or tests are named. Start by locating the Edit Mode settings and the common save path used for text formats; verify how the existing “Highlight line ending” setting is wired. Done means an opt-in setting removes all trailing whitespace on save across text formats, including a single trailing space.

Written by the indexing model from the issue text.