Polish the type definitions generator

[kainino0x]

A while back I wrote this document describing a potential project to fix a few bugs in the generator for WebGPU's TypeScript type definitions to make it more maintainable. I filed these as separate issues on bikeshed-to-ts, so this is essentially a tracking issue for those - see issues for details, see doc for summary.

https://docs.google.com/document/d/1MVPbUakXBKwvAd5CN3BJd2Ems5OhYhFt5pKpvrw8SN4/edit?usp=sharing&resourcekey=0-wdToK40p4UEnBKMNguYNmw

• IDL sequence should be Iterable, not Array darionco/bikeshed-to-ts#6
• [AllowShared] darionco/bikeshed-to-ts#8
• IDL object should translate as object darionco/bikeshed-to-ts#9
• declare var for WebIDL interface should always have new () darionco/bikeshed-to-ts#10
• new () should have return type darionco/bikeshed-to-ts#11
• Show dictionary default values in doc comments darionco/bikeshed-to-ts#12