Build script
ZoneGFX packages may have a build script named build.es along with the zonegfx.toml manifest file.
Re-run indicators
Indicate that a build script must re-run when an environment variable changes by tracing:
trace("zone::rerun-if-env-changed=VAR");
Indicate that a build script must re-run when a directory or file changes by tracing:
trace("zone::rerun-if-changed=path/to/file");