I reverse engineered GitHub’s slugging algorithm.
Somewhat based on #25 and #35.

To do that, I created two scripts:

* `generate-fixtures.mjs`, which generates a markdown file, in part
  from manual fixtures and in part on the Unicode General Categories,
  creates a gist, crawls the gist, removes it, and saves fixtures
  annotated with the expected result from GitHub
* `generate-regex.mjs`, which generates the regex that GitHub uses for
  characters to ignore.

The regex is about 2.5kb minzipped.
This increases the file size of this project a bit.
But matching GitHub is worth it in my opinion.
I also investigated regex `\p{}` classes in `/u` regexes. They work
mostly fine, with two caveats:
a) they don’t work everywhere, so would be a major release,
b) GitHub does not implement the same Unicode version as browsers.
I tested with Unicode 13 and 14, and they include characters that
GitHub handles differently.
In the end, GitHub’s algorithm is mostly fine: strip
non-alphanumericals, allow `-`, and turn ` ` (space) into `-`.

Finally, I removed the trim functionality, because it is not
implemented by GitHub.
To assert this, make a heading like so in a readme: `#  `.
This is a space encoded as a character reference, meaning that the
markdown does not see it as the whitespace between the `#` and the
content.
In fact, this makes it the content.
And GitHub creates a slug of `-` for it.

Further work: I think it would be nice to release this as is.
Then, afterwards, I’d like to modernize the project, add GH Actions
to generate the build, add types, and move to ESM.

/cc @Flet @jablkojablko

Closes GH-22.
Closes GH-25.
Closes GH-35.

Co-authored-by: Dan Flettre <flettre@gmail.com>
Co-authored-by: Jack Bates <jack@nottheoilrig.com>