feat(build): build.zig windows support

Tested using cross-compiling from linux:

    zig build -Dcross=true -Dtarget=x86_64-windows nvim_bin

Note: not fully functional without a runtime, which still has to be
fuddled with manually

Macos and windows builds require a recent zig 0.15+dev version
As this zig master branch is currently too much in flux, we can't make
our CI depend on zig master.

Revisit CI after zig 0.15 release or at least feature freeze.

.github/workflows/test.yml

@@ -209,7 +209,7 @@ jobs:
   zig-build:
     runs-on: ubuntu-24.04
     timeout-minutes: 45
-    name: build using zig build
+    name: build using zig build (linux)
     steps:
       - uses: actions/checkout@v4
       - uses: mlugg/setup-zig@v2