[Feature] Ability for themes to depend on plugins.

[Plaer1]

Before Requesting

• [X] I found no existing issue matching my feature request
• [X] This request is related to Millennium, and not a request for a specific theme/addon

Describe the feature you'd like!

I currently run the ATOMS skin and I'm going to put the ITAD integration into it's own plugin for everyone to use soon ʸᵃʸ. I would like to be able to have this plugin load when users install my theme, that way when you install my theme it still "feels" like the ITAD integration is a feature of the theme.

I know better discord has a dependency system for the plugins, I think having something similar will be good in the long run .I'm sure people will end up making libraries for their plugins eventually as well, I just think it would be cool to have it where themes could be dependent on plugins (but not the other way around, I do-not want to encourage a "walled garden" situation with the plugins).

Anything else?

No response