* Add minimal build.zig build script
The minimal `build.zig` script effectively can act as a (nicer?)
alternative to CMake; however, it is minimal in the sense, it currently
only includes the necessary ingredients to build `wasm3` with Zig
as the build system. As an added bonus though, you get the full
cross-compilation power of Zig for free.
Example invocation to build for the host:
```
zig build
```
To target `wasm32-wasi`:
```
zig build -Dtarget=wasm32-wasi
```
To target `aarch64-macos` from *anywhere*:
```
zig build -Dtarget=aarch64-macos
```
* Include Zig in build instructions
Jakub Konka