This PR adds a new field in the `MarkdownStyle` struct,
`heading_level_styles`, allowing, via the newly added function
`apply_heading_style` and struct `HeadingLevelStyles` to customize each
individual heading level in Markdown rendering/styling function.
Things like this should now be possible:
```rust
MarkdownStyle {
heading_level_styles: Some(HeadingLevelStyles {
h1: Some(TextStyleRefinement {
font_size: Some(rems(1.15).into()),
..Default::default()
}),
}),
..Default::default()
}
```
Release Notes:
- N/A
7e1b419243