customElements isn't defined there

This means less dom change on load

So that packages depending on it install it as well

node-resolve wasn't limited to the local node_modules and picked up the
dependency from another package on my machine.

So that rollup can do tree shaking and report import errors.

When specifying a package for code splitting, rollup requires the
packages to have a valid entry point for some reason.

Add an empty one.

So rollup doesn't have to convert them (and potentially fail)

In case the browser doesn't support custom elements and shadow dom we display
an error instead of the element.

This assumes the JS code executed thus far didn't lead to errors, so this only works
for semi-modern browers.