emscripten support

[joemarshall]

Hi, I'm a developer who has done a bunch of work making python things work on emscripten / webassembly.

I've already done work to add support for emscripten to urllib3 / requests, and off the back of that, I've been asked to add emscripten support to httpx.

In Emscripten, http support happens via the browser; so it handles all the protocol end of things. Given that, the obvious place to add support would be in httpx, as a new backend.

I'm posting this here to check if that's going to be okay, before I start doing the work.