feat(wasi): add wasm32-wasi as a compile target

[rioam2]

Supersedes #9

This PR adds support for cross-compiling to wasm32-wasi as a CMake target. It builds on top of the wasm32-wasi toolchain here, and more accurately, wasi-sdk.

The example contained in this PR exposes a WebAssembly interface following the reactor model. In other words, it is an example module that exposes functions that can be called in reaction to events on the host.

Usage information from README:

If you wish to expose parts of your library as a WebAssembly module, you can add the extern "C" __attribute__((export_name("<my_function_name>"))) annotation to any function you wish to expose in src/wasm/interface.cpp, and compile using the wasm32-wasi-clang preset. This will generate a minimal WebAssembly binary exposing your exported functions at build/<PRESET>/src/wasm/<Debug|Release|RelWithDebInfo>/lib<PROJECT_NAME>.wasm. This binary conforms to the WASI WebAssembly standard, so it can be utilized in any WASI-supporting runtime like wasmer.io or wasmtime.

A WebAssembly version of your CLI will also be available:

wasmtime run build/<PRESET>/src/cli/<Debug|Release|RelWithDebInfo>/<PROJECT_NAME>_cli

# Prints:
# <PROJECT_NAME> version: 0.0.1

Note

Exposed WebAssembly functions currently only accept parameters and return values of numerical type. This is how core WebAssembly files work, and without additional glue-code or binding generators like wit-bindgen, complex data-types cannot be sent over the runtime's ABI boundary. Complex data must be transferred via pointer and serialized bytes in exported/shared memory.

Acknowledgements:

• Catch2 uses libc stack_t and signal support, which has not yet been implemented in wasi-sdk. A custom port for this library is loaded from https://github.com/rioam2/vcpkg-wasm32-wasi
• fmtlib/fmt uses libc dup, which is not implemented in libc, and therefore wasi-sdk. As such, a custom port for this library is loaded from https://github.com/rioam2/vcpkg-wasm32-wasi
• The main registry for vcpkg is updated to https://github.com/rioam2/vcpkg-wasm32-wasi due to compatibility for fmt and catch2 with wasm32-wasi that has not been upstreamed.

[rioam2]

This was removed from .github/ci.yaml in 5efee87, so I removed it here in .github/workflows/ci.yaml as well. It fails due to missing libc++abi on x86

[rioam2]

Not necessary for this PR, but adds a nice way to debug issues in CI.
Can be removed and put up as a separate PR.