chore: project scaffold and build wiring

Initial Zig 0.16 build script wiring the public `zbench` module, the
`zbench_build` helper module for downstream `build.zig` files, an
internal `test` step, and an `example` step that compiles and runs the
bundled benchmark suite.

.gitignore

@@ -0,0 +1,2 @@
+.zig-cache/
+zig-out/