Additional error checking in Compression API

[fhanau]

Perform more error checking for DecompressionStream, which requires adding a new compat flag.

[dom96]

See comment, but otherwise changes look good so accepting to unblock.

[dom96]

I'm somewhat suspicious of this, mainly because I haven't seen feature flags be accessed like this before. I suppose if it works it's fine, but iirc jsg will get you the feature flags if you just put them in the method (CompatibilityFlags::Reader featureFlags) and I see a lot of other code doing it this way so might be better to be consistent.

[fhanau]

Thanks for the suggestion, switched to getting the compat flags from JSG.
I found the old approach in a global scope method (ServiceWorkerGlobalScope::request) and was initially going to use it in pumpOnce() directly, but since the flags check is now in the constructor it is certainly cleaner to have them in the function parameters.