Use ↩ as default params.FootnoteReturnLinkContents

[adiabatic]

This changes the default params.FootnoteReturnLinkContents from <sup>[return]</sup> to <span aria-label='Return'>↩\ufe0e</span>.

It’s very common to use ↩ in footnote links. However, some platforms like iOS and iPadOS choose to use emoji presentation for this particular character. This leads to lots of blogs, by default, looking silly on portable Apple gizmos, as described in jgm/pandoc#5469. By switching to a return arrow with a disable-emojification variation selector, we get blackfriday to do the right thing by default.

Additionally, ↩ is more language-agnostic than “return” is, so blackfriday will work better out of the box for more people.

[dmitshur]

The documentation for the HTMLRendererParameters.FootnoteReturnLinkContents field also needs to be updated. It currently describes the old default value:

$ go doc github.com/russross/blackfriday/v2 HTMLRendererParameters.FootnoteReturnLinkContents
package blackfriday // import "github.com/russross/blackfriday/v2"

type HTMLRendererParameters struct {
    // Show this text inside the <a> tag for a footnote return link, if the
    // HTML_FOOTNOTE_RETURN_LINKS flag is enabled. If blank, the string
    // <sup>[return]</sup> is used.
    FootnoteReturnLinkContents string

    // ... other fields elided ...
}