UI buttons and hotkeys to easy edit text in markdown mode

### What

Add UI buttons and hotkeys to easily edit some text in markdown mode. For example, just select some text and click the bold UI button or hotkey for bold action. So at right and left selected text add ' ** ' to do text bolder as in markdown style. It would be so good for the all-supported markdown style and for all supported os. Now we have only for a checklist.

### Why

This is easy and quick to write some text. We don't need to distract from remembering how to format text and what symbols we need to use or just waste our time.

### How

This functionality we can see everywhere. Even where I write now, like in the GitHub issue description editor. It's a very good example.

Start by locating the markdown-mode editor and its existing checklist control, then review how selection and keyboard actions are handled. Define the supported formatting actions and platform behavior from the requested scope; done means the agreed buttons and hotkeys work on selected text across supported operating systems.

Written by the indexing model from the issue text.