Add support for data uri's in fetch

[ronag]

https://www.npmjs.com/package/data-uri-to-buffer

[ronag]

Also find the relevant section in the spec https://fetch.spec.whatwg.org

[ronag]

@szmarczak @Ethan-Arrowood @mcollina Looking at the spec "scheme fetch" is only invoked for requests with mode "navigate". However, it is not possible to create requests with that mode.

The only place where schemeFetch is invoked is at 11. https://fetch.spec.whatwg.org/#main-fetch

request’s mode is "navigate" or "websocket"

1. Set request’s response tainting to "basic".
2. Return the result of running scheme fetch given fetchParams.

However it doesn't seem to be possible to create a request with mode set to "navigate" given https://fetch.spec.whatwg.org/#request-create

So I guess we only support http fetch.