Support kerning groups in SVG in- and export.

[thoka]

Since hkern in SVG may contain groups of targets in u1 and u2 attributes (delimited by ,), kerning groups could be supported in SVG in- and exports.

This behaviour should be default for imports and optional for exports.

[mattlag]

https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/u1

I guess I didn't dig deep enough into the SVG Font spec - you are right! The danger with everything SVG Font related, though, is that it's totally deprecated, so support for it quite possibly could be non-existent, or not complete.

I agree with having an option for export. Some online font converters may not know about the comma separated u1 and u2 attributes.

Ambitious goal for import: even if they aren't grouped, infer grouping by trying to collapse down common pairs.