Add an option to include the shim script as defer instead of async

[JanPokorny]

Although the es-modules-shims script recommends including it as async in their README, this can lead to bugs as on browsers that support executing type="module" scripts out of the box, the async script might not execute before the module, leading to the importShim function not being available: guybedford/es-module-shims#385

This can be mitigated by changing async to defer when including the shim -- deferred scripts go into the same queue as modules, thus it guarantees that the shim will be executed first, before the module.

[JanPokorny]

As explained in the linked issue, it actually only makes sense to do this if the script needs to rely on importShim (which is only when there are no static imports to trigger the polyfilling mechanism). In the general case async is better than defer, but still having an option to choose between async/defer would be nice to allow for this edge case

[dhh]

We're dropping the shim in version 2, so this is no longer relevant.