This has proper support for rollup while providing the same API.

This is what open-wc recommends now and uses in their @open-wc/testing package

Versions: Lit-Element to 2.4.0 and Lit-HTML to 1.3.0

This fixes a typescript problem and allows us to enable type checking
in the typescript-example package (by removing the ts-nocheck comment)

This is all very basic, building works, testing works,
but there are type checking errors which I don't know how
to fix.

Improvements welcome.

See #41