add a newline at the end of each file (cross-platform compatibility)

### ⚠️ 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

1. [Ending a text file in a newline character is a Unix convention](https://unix.stackexchange.com/questions/18743/whats-the-point-in-adding-a-new-line-to-the-end-of-a-file) which native text editors follow.
2. Markor does not add the newline character when saving, which will for instance result in redirected output (`>>` in Bash) being written on an existing, non-empty line. I edit some of my files using both Markor and various Linux software (Termux on Android, desktop Linux); URLs in particular can be time-consuming to fix (e.g. `https://www.youtube.com/watch?v=jNQXAC9IVRw` may change to `https://www.youtube.com/watch?v=jNQXAC9IVRwab`).
3. Can you ensure that all files modified by Markor end in the newline character?

### Information

Android version: LineageOS 23 (Android 16)
Device (phone model): Oneplus 8T
App Version: v2.16.1

### Source

F-Droid

### Format / File type

Something else (-> Additional info)

### Additional info / Log

```shell
All but the binary file formats seem to be affected.

For now I simply edit all text files shared with Markor using vim on the desktop — I can't conveniently write to the Markor-modified files with say `tee` or `cat`, without scripting in some additional logic.
```

Start by locating Markor’s save logic for text and binary file formats, then inspect how file content is written. Confirm which non-binary formats are affected and define completion as ensuring every file Markor modifies ends with a newline without changing binary handling; add or run relevant save tests if the project provides them.

Written by the indexing model from the issue text.