Provide a semver-stable guarantee (1.0 release)

Bazel 8 had strong LTS and stability guarantees.

with the replacement in Bazel 9, we now have a version 0.6 of rules_shell which implies that it's unstable - semver says that version 0.7 is free to have arbitrary breaking changes.

For teams that make dependency decisions based on this, a 1.0 release would be a better signal of stability.