Extract Phosphor icons from a single font or sprite file

Problem/Motivation

In #3539895: Support the Phosphor icon library used by core → , we added over 1,500 SVG files to Mercury: the complete Phosphor icon library.

That's a lot of damn files. It would be a lot better if we could ship a single file (a font? an SVG with sprites? something else?) with all those icons. This issue is to find that better way. The one proviso here is that the icon IDs must remain the same! Or existing usages of the icons will probably break.