[WIP / Discussion] Allow for non-lit specific implementation

[KonnorRogers]

What I did

1. Moved the adoptStyles call to be a hook

EDIT: will also need to move static elementStyles out as well.

Why?

Ideally I would love to be able to use this mixin with any web component, and as far as I can tell this is really the only Lit-specific portion.

Technically this PR doesn't solve the problem, but if the base mixin has a hook to adopt styles, we could get a separate entrypoint for the Mixin for Lit-specific implementations with the actual adoptStyles implementation.

I didn't do it here because I figured I'd open it up to discussion first if this is a use-case you all would like to support and making sure I didn't miss anything along the way.

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[thepassle]

The ScopedElementsMixin is indeed built for usage with LitElement, and depends on overriding lits createRenderRoot lifecycle method. Moving the adoptStyles function isn't going to be enough to make the mixin compatible with non-Lit elements, unless they also implement a similar lifecycle hook. ~I also don't think the adoptStyles function is actually an issue here, I think it would just noop when used with non-lit elements.~ Nope, looks like it does error on non-lit elements.