Rename the factory as available on window to window.trustedTypes.

[koto]

The spec trend is now to start with lowercase, so window.trustedTypes instead of window.TrustedTypes.

We should perform the rename as the last step after building + speccing all existing features, such that window.trustedTypes has stable features, unlike the current API that is under active development.

Note to authors: Please feature check before using the API i.e. check that window.TrustedTypes.createPolicy exists before calling it.

[mikesamuel]

Re the polyfill, I wonder whether making it es-shim API compatible would ease some of these migration issues.

[koto]

Chrome implementation tracked in https://bugs.chromium.org/p/chromium/issues/detail?id=991944.