build: add Emscripten/WASM support

[Rawan10101]

Fixes: #8

[torque]

Looks like build.zig needs to be hit with zig fmt. I have no way to test this, but if you've tested this with neovim and it works for you I'm happy to merge it.

[Rawan10101]

@torque yes, I tested this with Neovim's Zig build system targeting wasm32-emscripten. it builds successfully and it produces a libuv.a static library. I will fix the formatting with zig fmt and address the --search-prefix suggestion now.

[torque]

Can you confirm the correct usage is now something like

zig build -Dtarget=wasm32-emscripten --search-prefix $EMSDK/upstream/emscripten/cache/sysroot

In projects that use this as a dependency, do they need to make any changes to their build.zig to support this, or does --search-prefix get propagated automagically?

[Rawan10101]

Yes, it is the correct usage. For most projects that use libuv as a dependency, no changes are needed in their build.zig as --search-prefix is propagated automatically to all dependencies.
The only exception is projects that cross-compile two targets simultaneously like Neovim, as it builds a native host tool and a WASM binary in the same build. In that case the project needs to handle the sysroot explicitly in its own build.zig to avoid applying it to the host build. But that is the dependent project's concern not libuv's.

[torque]

thanks!