Fix running of test and tools on windows and macos (#6)

* Run build tools on windows also to help catch regressions

* Overhaul and modularize build.zig

Also ensure one can build on and for windows
Awaiting testing to ensure test run on windows

resolves #3

* tools: disable mplay tool

It isn't cross compilable to windows due to the use of sys/wait.h
which is a posix system header

It also doesn't compile on musl libc because mplay modifies stdin and
stdout which are const in musl libc

The darling issue has been splited from #3 into #7

.github/workflows/ci.yml

@@ -62,8 +62,10 @@ jobs:
           version: ${{ matrix.zig-version }}
           use-cache: false
 
-      - name: Run `build tools on macos`
-        run: zig build tools -Dtarget=x86_64-macos-none --summary all
+      - name: Run `build tools on macos and windows`
+        run: |
+          zig build tools -Dtarget=x86_64-macos --summary all
+          zig build tools -Dtarget=x86_64-windows --summary all
 
   test:
     strategy: