keep numbered headings

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

- [x] This is a **bug**. Not a [question or feature request](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 bug is still 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 settings are not touched. In case the issue is resolved there, you don't need to create a bug report. The change will be part of the next Markor update.)

### Description

numbered heading:

```md
# 1. some title
```

### Steps to reproduce

currently, when i want to convert this to a heading

```
1. some title
```

then markor drops the numbering and gives

```
# some title
```

### Information

Android version:
Device: Gigaset GS110
App Version:

### Source

Google Play

### Format / File type

Markdown

### Additional info / Log

```shell
-
```

Reproduce the conversion in Markor's Markdown editor using a line such as `1. some title`, then trace the entry point that turns numbered text into a heading. Confirm the current output and identify the relevant tests or editor code before changing anything. Done means converting the line preserves the `1.` prefix in `# 1. some title`.

Written by the indexing model from the issue text.