Add async build support for the Web platform (asyncify)

[adamscott]

Note

This PR is not needed per se now, but it will prove useful for integrating features that require deep asynchronicity when the codebase expects an immediate response. I think integrating WebGPU is such a case.

This PR adds asynchronous build support to the Web platform by introducing 2 build variables:

Build variable	Description	Default value	Values
async	Enable synchronous C++ interaction with asynchronous JavaScript. Produces bigger binaries.	no	no, yes
async_method	Set the async method to use. Only applies if async=yes.	asyncify	asyncify, jspi

See Asynchronous Code from the emscripten docs for more details about async_method.