std.mem.writeVarPackedInt: handle write_size == 0

[clickingbuttons]

Also move example comments into tests.

[clickingbuttons]

Windows failure seems to be a RemoveDir flake.

Main test suite...
test
+- test-cli
   +- RemoveDir C:\Users\ci\george4\_work\zig\zig\zig-cache\tmp\747baf9cde4f97ee
      +- zig build test failure
error: Unexpected
error: the following command exited with code 1 (expected exited with code 0):
cd C:\Users\ci\george4\_work\zig\zig\zig-cache\tmp\747baf9cde4f97ee && C:\Users\ci\george4\_work\zig\zig\build-release\stage3-release\bin\zig.exe build test 
Build Summary: 6159/6432 steps succeeded; 269 skipped; 1 failed; 418[96](https://github.com/ziglang/zig/actions/runs/8803469365/job/24161668086?pr=19745#step:3:97)/44139 tests passed; 2243 skipped (disable with --summary none)
test transitive failure
+- test-cli transitive failure
   +- RemoveDir C:\Users\ci\george4\_work\zig\zig\zig-cache\tmp\747baf9cde4f[97](https://github.com/ziglang/zig/actions/runs/8803469365/job/24161668086?pr=19745#step:3:98)ee transitive failure