Added so that GFM has support for creating line breaks by prefixing the line feed with a backslash.

I didn't add any render* function for the line breaks, since newlines rendered by renderText are never tokenized either. Is that okay?

On the side, here are some issues I've found:

• Maybe trim the lines in the Markdown files in the repo to 120 characters per line? It's kinda annoying to view them.

• The PHPDoc uses the @inheritDoc tag, not @inheritdocs, which is never mentioned in the docs, VSCode also doesn't highlight it. Dunno about PHPDoc itself, because I never used the software itself (Intellisense FTW). Intelephense (language server for PHP) does actually understand the tag, but that's only because it doesn't seem to check if there's a space after @inheritDoc, so even @inheritDocRandomStuffHere will work. I suspect the tag is not valid then.

• composer-install outputs:

  Package facebook/xhprof is abandoned, you should avoid using it. Use lox/xhprof instead.
  Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.