Implement integrity proposal

[guybedford]

It may be worth considering implementing the "integrity" property in import maps at some point, per https://github.com/guybedford/import-maps-extensions#integrity as was implemented in SystemJS in systemjs/systemjs#2229.

This would allow import maps in es-module-shims to define integrity and fully guarantee those module execution commitments in polyfill mode.

The risk of implementation is that passthrough mode would not get the integrity feature - so that the 70% passthrough case effectively becomes less secure. This could be mitigated by making the feature a polyfillEnable feature, which when enabled fully disables passthrough with the performance cost justified by the security benefit.