feat: add alpha and beta release channel support (#33098)
## Summary
- Add `Alpha` and `Beta` variants to `ReleaseChannel` enum with full
serialization/deserialization support
- Auto-detect channel from version string prerelease tag (e.g.
`2.8.0-alpha.0`) as fallback in `version.rs`, eliminating need for new
env vars or CI changes for tag builds
- Enable `deno upgrade alpha` / `deno upgrade beta` commands, with
downloads via GitHub Releases (same as stable)
- Add `--alpha` / `--beta` handlers to version bump script that fall
through to CI cache and release notes updates (unlike RC which
early-exits)
- Add `alpha` and `beta` options to `version_bump.yml` workflow dispatch
- Make `post_publish.yml` channel-aware: writes to
`release-alpha-latest.txt` / `release-beta-latest.txt` and skips
`version_greater_latest` check for non-stable releases
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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
