Uses the provided name for a PHP Markdown Extra Footnote reference when it is
not an incorrectly ordered number value, such as: [^named]. The reference names
were previously ignored and replaced with ordered index integers. According to:
https://michelf.ca/projects/php-markdown/extra/#footnotes
> Names can contain any character valid within an id attribute in HTML.
This makes three changes to backlinks:
- Inserts a space before `<a href='…'>↩</a>` to make it look better
- Uses hexadecimal references (`&#x…;`) instead of decimal references for clarity
- Adds U+FE0E VARIATION SELECTOR 15 to the ↩ to suppress emojification on both iOS and Edge on Windows
If variation selector 15 isn't appended to the ↩, then the return arrow will show up as an emoji on iOS and Edge for Windows. The Pandoc project has already run into this quirk as documented on https://github.com/jgm/pandoc/issues/5469 and they've pushed a change for their Markdown processor already.
