Theme Presets

[12944qwerty]

Adds theme presets.

I've tested with a modification of the Catppuccin theme.

Presets are specified in the manifest

{
    ...
    "presets": [
        {
            "label": "flavorname",
            "path": "relative/path/to/flavor",
            "default": false
        }
    ]
}

[asportnoy]

Naming needs changing per internal discussion a few weeks ago