Add ability to disable themes from ui_skins.themes.yml definitions

Problem/Motivation

When dealing with sub-themes generating from a theme in the contrib space that could have a lot of themes off-the-shelf (i.e. DaisyUI: https://daisyui.com/docs/themes/), it would be useful to have the ability to disable the themes we don't want to see/use in the sub-theme.

User interface changes

Add and manage a new key (example: disable) in the ThemeDefinition.php