Support browser/WebAssembly build

[jonnor]

MicroPython can run in the browser using the WebAssembly/WASM port. This would enable to use emlearn-micropython in the browser, be it for small demos, educational usage or providing "virtual" hardware emulator for an embedded device. Or it could actually be used emlearn to build a compact web application which incorporates machine learning (though that is perhaps better done with emlearn C library combined with JavaScript).

Now that we can support building as external C module (ref #18), this is at least theoretically possible.

[jonnor]

Tulip Creative Computer is an embedded device made with MicroPython. It can be ran on the web using MicroPython WASM: https://tulip.computer/run/
Their build is open source, and might be relevant in how they achieved this. https://github.com/shorepine/tulipcc/blob/main/tulip/web/Makefile