Highlight in-search results

### ⚠️ 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. One of the most urgent feature for serious text processing: being able to find items you searched for by using integrated search. I use Markor predominantly for doing html/css/js drafts or quick feature drafts, stuff like that. Now it happened multiple times (enough it seems to stop here and talk about it :-)) I needed to find certain strings in minified codeblocks or just large blocks of continuous code not interrupted by any paragraphs. To find the search result without it being highlighted is basically impossible resp. equals going through the code manually. So, just as some sort of suggestion for your backlog, next time someone touches search, perhaps it would be possible and feasible to implement highlight function without having to refacture half the search code. For me, it would be a significant improvement and something I'm missing most right now. I could imagine for more people appreciating it.

Thanks for reading!

Edit: I thought it was clear, but maybe it's not: all the above written refers to edit view in plain text/html mode. It might also apply to edit modes in other file formats.

### Information

Android version: n/a
Device: n/a
App Version: n/a

### Source

GitHub Releases

### Format / File type

Not specific

### Additional info / Log

```shell
-
```

Start in Markor's edit view and integrated search behavior, reproducing a search for text in a large plain-text or HTML code block. Trace the search entry point and editor rendering, then verify that every matching string is visibly highlighted in edit view without disrupting other file formats.

Written by the indexing model from the issue text.