[browser] Allow access to asset data after `dotnet.download()`

[carlreinke]

dotnet.download() can be used to download the assets, but there seems to be no way to access the downloaded assets.

Perhaps download() could give access to getConfig()?

Background
I am running .NET in an audio worklet, which has no ability to fetch() assets. Right now I am loading the assets by proxying the fetch requests through the message port of the audio worklet (except for the .js assets, which are statically imported since an audio worklet cannot use import()). The main thread has already downloaded the assets for it's own instance of the runtime, so instead of fetching all the assets twice (hopefully the second time from cache, but no guarantees), I'd like to just download the assets once, send a copy of the assets across the message port (which will be used to instantiate the runtime there), and then instantiate the runtime in the main thread (which consumes the buffers of the assets, so I cannot get them after dotnet.create() via getConfig()).