[Code blocks] Magic Comments to mark highlight ranges

Context

While researching other static site generators in #213, I discovered a really cool syntax for highlighting code block lines in Docusaurus.

This PR adds the ability to highlight code block lines using "magic comments". Primer Spec will recognize code block lines with comments primer-spec-highlight-start and primer-spec-highlight-end, and will automatically ensure that the lines in between are highlighted!

This only works with code blocks with syntax-highlighting, and only for pages built using Jekyll.

Validation

Check out the demo preview URL: https://preview.sesh.rs/previews/eecs485staff/primer-spec/218/demo/enhanced-code-blocks.html#markdown-source-for-magic-comments-6

It contains the following code block:

```python
x = {
  # primer-spec-highlight-start
  # This dict refers John, whose age is 30
  "name": "John",
  "age": 30,
  # primer-spec-highlight-end
  "cars": [
    {"model": "BMW 230", "mpg": 27.5},
    {"model": "Ford Edge", "mpg": 24.1}
  ]
}



Which renders without the magic comments, and with the correct lines highlighted:
> <img width="1017" alt="image" src="https://user-images.githubusercontent.com/12139762/196071672-13d21719-4b79-46dd-8126-00e259ed58a8.png#gh-light-mode-only">
> <img width="1015" alt="image" src="https://user-images.githubusercontent.com/12139762/196071635-67fa956e-f27d-4cbb-a3ab-36bd4c5d0773.png#gh-dark-mode-only">

I also locally tested the following scenarios:
- HTML and JS code blocks (with different comment syntax)
- Code blocks with both magic comments *and* a `data-highlight` attribute

eecs485staff / primer-spec

[Code blocks] Magic Comments to mark highlight ranges #218

Context

Validation