Copy over auto-tagging functionality from sbt-guardrail

[blast-hardcheese]

Adds a workflow that automatically creates a tag like v${MAJOR}.${MINOR}.${PATCH} based on the semver version of the guardrail core library, as depended on in build.gradle.

This will automatically release and tag when PRs like #33 are merged to master, opening the opportunity for automatic CI/CD like guardrail-dev/sbt-guardrail#84.

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