Add ability to add themes and fonts via "Open in.." Extension

As suggested in https://github.com/blinksh/fonts/issues/1, please add ability to install fonts and themes via "Open in.." Extension.

Suggested workflow: assume .js files are themes and have fonts installed via self-contained .zip file with both .css and .ttf files included in the archive.

Possible concerns: may conflict with future workflow for SCP.

Start by reading the existing “Open in…” extension handling and how imported files are routed. Check the proposed formats: .js files for themes and self-contained .zip archives containing .css and .ttf files for fonts. Done means both kinds of imports install correctly without blocking the future SCP workflow.

Written by the indexing model from the issue text.