Markdown Footnote Syntax highlighting, insert and view support

### ⚠️ 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. Syntax Highlighting for Markdown footnotes (standard and inline footnotes)
* `This is a Markdown sentence.[ˆ1]`
`[ˆ1]: footnote`
* `This is a Markdown sentence.ˆ[inline footnote]`
3. maybe a footnote insert button
4. rendering of footnotes as footnotes in the View

### Information

Android version: 16
Device: Lenovo Yoga Tab
App Version: v2.15.2

### Source

F-Droid

### Format / File type

Markdown

### Additional info / Log

```shell
-
```

Start by locating Markor's Markdown syntax-highlighting, insert-button, and View/rendering entry points. Check how standard and inline footnotes are currently parsed or displayed, then verify that highlighting, insertion, and rendered footnotes work for both examples in the issue.

Written by the indexing model from the issue text.