build: use whiskers

[uncenter]

Adds Whiskers, Catppuccin's internal templating tool, to build the themes. Instead of editing the .yaml files directly, edit the tabby.tera file and then run just build to build the output themes. You will need Whiskers (linked above) and optionally Just installed. I've also updated the usage instructions and cleaned up the image assets.

Note: I've attempted to match the themes exactly as they are right now. The remaining differences you can see are mismatched colors from before Whiskers (e.g. mocha base in a macchiato theme).