Add hotkeys for text formatting in Markdown editor
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### Description
When I type some text in the editor, it's not very useful to move my hand to grab my mouse just to click to a button to format text.

### Proposal
Add hotkeys when focus is in the Markdown editor (`md-area`):
| Action | Suggested hotkey | Current status |
| -------- | -------- | -------- |
| Toggle `Write/Preview` | <kbd>CTRL</kbd> + <kbd>SHIFT</kbd> + <kbd>P</kbd> | **Implemented** |
| Add **bold** text | <kbd>CTRL</kbd> + <kbd>b</kbd> | [**Implemented**](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40328) |
| Add *italic* text | <kbd>CTRL</kbd> + <kbd>i</kbd> | [**Implemented**](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40328) |
| Add `inline-code` text (around selected text) | <kbd>`</kbd> | **Implemented** |
| Insert a quote | <kbd>CTRL</kbd> + <kbd>></kbd> | *Not yet implemented* |
| Add a task list | <kbd>CTRL</kbd> + <kbd>u</kbd> or <kbd>CTRL</kbd> + <kbd>o</kbd> | *Not yet implemented* |
| Add a link | <kbd>CTRL</kbd> + <kbd>k</kbd> | [**Implemented**](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40328) |
issue