Implement hiding code lines

[sicikh]

Official mdBook documentation describes how to hide specific code lines in block via following configuration:

[output.html.code.hidelines]
typst = "% "

or by code block specifically:

```typ,hidelines=^^^
^^^I will be hidden.
I will be visible.
```

As preprocessor modifies original code blocks, this won't work as is. This PR resolves that:

Lines hidden by default

Visible hidden lines

CHANGES:

1. Added PreproccessSettings::hide_lines field that is read from output.html.code.hidelines field in config.
2. Added reading hidelines=... option from code block, this option takes precedence over PreproccessSettings::hide_lines.
3. Markers are removed when highlighting and rendering, <span class="boring"> is added when highlighting. This span automatically enables toggling lines visibility in mdBook.
4. Updated README.md and example book accordingly.

[sicikh]

@sitandr PR is ready for a review, I've updated the branch to the current main.