feat: Helix Selection Renderer in Markdown (.multicursor file type)

[nik-rev]

This PR adds support for rendering Helix selections in markdown components:

`copy_selection_on_prev_line`

Copies the current primary selection to the first previous line long enough to accomodate it.

# Examples

The selection is copied from line 2 to line 1.

Before:

```multicursor
This is text #[|on line 1]#.
This is text on line 2.
```

Command: `C`

After:

```multicursor
This is text #(|on line 1)#.
This is text #[|on line 2]#.
```

Renders as:

It'll nicely complement #997 when that gets merged.

Here is the kind of workflow I'm envisioning:

• All Helix help files will be written in markdown, using these blocks.
• The :help completer will show these Helix selections when you press tab to go see different commands. You'll be able to access the documentation on each command just by holding tab
• We'll also use this to generate integration tests that automatically run. So imagine this, you write documentation for a Helix command but you also the documentation won't become outdated because it's automatically tested
• We can then turn these files into HTML to render on the website (like what was my dream from the beginning 😎 )

This PR just implements the renderer. The other stuff can be added in a different PR :-)

Tracked by #12714