Nim is a systems programming language (included in the highlight.js
`system` group), and we're quite happily using `mdBook` in several of
our documentation projects starting with our [style
guide](https://status-im.github.io/nim-style-guide/).
While we can maintain our own highlight.js, including `Nim` in the
default distribution would allow us to promote more mdBook usage in the
Nim community at the cost of a ~2kb increase in the `highlight.js` size.
Allows for special styles to call them out since they're different than
normal text and different than code. They can make use of styles they
inherit for font style and weight.
Notes on changes:
- Added new CSS variables for reused elements
- The font-* rules are separate for each aspect so that they can inherit
bold/italic/etc
Closes https://github.com/rust-lang/mdBook/issues/1813
