add option to display first line of text in md files in the File Description Format of the file browser

### ⚠️ 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)_.
- [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. Add "firstline" option to Settings/Other/File Description Format that will display the first line of text from the file in the description line under the filename in the file browser.
2. Add support for a new line character "\n" to allow for multi-line descriptions.

A "File description format" in Settings/Other like this:

```
firstline \n FS yyyy MMM dd:hh:ss aa
```

would display a description like this under the file in the file browser:

```
First line of text in the .md file
30KB 2020 Mar 20 11:24:52 PM
```

### Information

Android version: 14
Device: Google Pixel 6 Pro
App Version: v2.12.5 (154)

### Source

GitHub Releases

### Format / File type

Markdown

### Additional info / Log

```shell
-
```

Start by locating the Settings/Other File Description Format implementation and the file-browser code that renders descriptions. Trace how existing format fields are parsed, then verify the requested first-line Markdown value and escaped newline behavior in the browser. Done means both options produce the example's two-line description without breaking existing formats.

Written by the indexing model from the issue text.