Support prerelease versions in the release pipeline

[lbebber]

Summary

• release-version.js: normalizeVersion now accepts X.Y.Z-<prerelease> versions (e.g. 1.2.3-alpha.1). Returns prerelease and distTag fields (distTag is derived from the first segment: alpha from alpha.1).
• publish-packages.js: Passes --tag <distTag> to npm publish for prereleases so they don't become latest.
• release-workflow.js: Prereleases can be dispatched from any branch (not just main). Stable releases still require main. Skips the assertOriginMainSha check for prereleases.
• release.yml: tag_and_dispatch job now runs from any branch. The JS scripts enforce the main-only constraint for stable releases.

Usage

Dispatch the Release workflow from any branch with a version like 1.2.3-alpha.1. It will:

1. Tag and build as usual
2. Publish to npm under --tag alpha (not latest)
3. Users can test with npm i -g @customerio/cli@alpha

Test plan

• Existing tests updated and passing
• Dispatch a dry-run from a non-main branch with a prerelease version
• Publish an actual alpha release and verify npm i -g @customerio/cli@alpha works