to `markup.horizontal-rule.gfm` and add styles to all eight built-in syntax themes to account for it.
All the old GFM scopes are just named however they needed to be named in order to produce syntax highlighting to the grammar author, semantics be damned. Instead, we're making sure the semantics are correct, then adapting the syntax themes accordingly.
Quite proud of this one — it uses an outer parser to identify front-matter and
highlight it properly, then injects the Markdown parser into everything that
isn't front matter.
The scope names are quite different — because the scope names in the legacy
Markdown grammar are awful — so I adapted the eight built-in syntax themes so
that there are no major style differences.
Update of the default syntax themes to implement [naming conventions](https://github.com/atom/flight-manual.atom.io/pull/564) for syntax scopes.
Adds the [template](https://github.com/atom/apm/pull/883) to each theme with custom colors, to accommodate the naming conventions. There should be no compatibility break with existing grammars.
As naming conventions are implemented in more language grammars, their old specific stylesheets can be retired.
