feat request: textwidth option

### ⚠️ 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

Hi, I am a vim user and plan to edit notes in vim and markor. Vim has a setting called `textwidth`, the point of which is to break a string when it reaches a certain number of characters. I haven't found a similar setting in markor. There is a _wrap words_ setting in _file settings_, but it doesn't break lines, it just changes how they are displayed.

### Information

Android version: 12
Device: boox tab ultra c pro
App Version: v2.12.5

### Source

Google Play

### Format / File type

Markdown

### Additional info / Log

_No response_

The issue names no files, tests, or entry points; start by locating Markor's file settings and the existing “wrap words” editor behavior. Determine how a configurable text width should apply to Markdown editing, and consider the work complete when lines break at the selected character limit rather than only changing display wrapping.

Written by the indexing model from the issue text.