Load color schemes from individual files

[joshka]

Is your feature request related to a problem? Please describe.
I recently added VHS format color schemes to the iterm2 color schemes repo at https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/vhs (which is the upstream from the windowsterminalthemes.dev VHS uses.

It would be convenient to load the themes from individual files rather than a json string in each tape or by editing a large json blob. This would make it easier to write multiple tapes and make custom themes for a product's screenshots / vids.

Describe the solution you'd like
Transition to using individual theme files and source these from the upstream source.

Describe alternatives you've considered
Continue to use the single file approach but update it (#377 does this)

Additional context