Asciidoc :icons implementation

### ⚠️ 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. I would like for implementation of asciidoc icons `icon:heart[role=red]`. That would help greatly, since icons can be colored and have many great inline formating features.

It would great if the `:icons: font` mode worked, but having at least the `:icons: svg` + `:iconsdir: ./icons` mode worked.

### Information

Android version: 11
Device: xiaomi Mi 10 lite 5g
App Version: v2.11.1(148)

### Source

F-Droid

### Format / File type

Something else (-> Additional info)

### Additional info / Log

```shell
- it is for Asciidoc format
- currently we can use the svg image:{icon_name} as a workaround, but it lacks colouring and using it inline is not straightforward.
```