Add webidl iterable support

[RaoulHC]

Was trying to figure out how to list files in OPFS before I realised these methods were just missing from web-sys.

Added some tests that use these and more generally test some of the file system API, only tested it with geckodriver but it should be well supported across all browsers.

Also added some flags so that the optional stream module should get generated in docs.rs too, wasn't obvious it existed, and it's somewhat needed to make use of the AsyncIterator that these methods return.