Port indent feature of footnote function form CaSILE

[alerque]

While working on #372 for @jtauber I ran across something that didn't work that I thought should: namely lskip/rskip in footnotes. It turns out I've been using a re-implementation of the \footnote{} command from CaSILE that has an indent= option that makes alignments like this possible:

This should probably be ported over because it seems generally useful ... or at least some way of controlling the layout of footnotes outside of the material vbox that gets stuff stripped off it.

[Omikhleia]

Please clarify.

[alerque]

The hanging indent style here is not the default behavior, nor ever supported in plain SILE at the time I wrote this issue. It might be possible to uses the new paragraph shape tooling you've added since to get a better implementation than the CaSILE one shown here that uses negative width boxes and reboxing.

[Omikhleia]

Almost forgot my question above. Thanks for the answer too!
My own implementation of footnotes in the resilient collection also has the capability of adopting that type of formatting, though likely using a different technique. It does use lskip/rskip in footnotes, I haven't met any problem so far (but maybe there's a catch there?)