.diff file format highlighting

### ⚠️ 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. Using Markor for code editing, it would be useful to have .diff format syntax highlighting
2.
3.

### Information

Android version: 16
Device (phone model): galaxy a15
App Version: 2.15.2

### Source

Google Play

### Format / File type

Something else (-> Additional info)

### Additional info / Log

```shell
-
```

No file or test is named in the issue. Start by finding Markor's code-editor syntax-highlighting configuration and reproduce opening a .diff file; done means diff syntax is visibly highlighted without changing other formats.

Written by the indexing model from the issue text.