Allow Passing Shadow Root as Sheet Target

[joealden]

In @types/styled-components, the target prop of StyleSheetManager allows passing a ShadowRoot (see here).

As I'm not familiar with this codebase, I started from the change to IStyleSheetManager, then fixed any type errors that surfaced. @quantizor please let me know if you see any problems with what I've done here, or if you know of a reason why the ShadowRoot option wasn't carried over into v6!

[joealden]

@quantizor sounds good to me! Do you think it should live in /src/types.ts or /src/sheet/types.ts?

[quantizor]

@quantizor sounds good to me! Do you think it should live in /src/types.ts or /src/sheet/types.ts?

src/types is good :)