build,doc,tools: support alpha releases

[RafaelGSS]

doc: document the alpha release process

Document how to prepare, version, tag and promote the Alpha releases
introduced by the new release schedule, starting with Node.js 27, and
how the first Alpha and semver-major releases are prepared with
git node release.

Refs: https://github.com/nodejs/Release/issues/1187

build,tools: support alpha versions in release tooling

Alpha releases are versioned vN.0.0-alpha.X.Y.Z. Add a --full flag to
tools/getnodeversion.py that includes the alpha tag on release builds
and use it for the release artifact names on both Unix and Windows,
keeping the MSI ProductVersion numeric. Make the tag check in
tools/release.sh and the release tests accept alpha versions too.

Refs: https://github.com/nodejs/Release/issues/1187

tools: support alpha releases in workflows

- Lint alpha release proposal branches and expect the "(Alpha)"
  release commit title.
- Accept alpha anchors and SEMVER-MAJOR commits when checking the
  proposal commit list.
- Add an optional release-version input to create-release-proposal,
  passed to git node release as --newVersion.
- Check the documented versions against the changelogs on alpha
  release lines too.
- Skip the post-release actions for alpha releases.
- Only create the major release reminder in February, as there is a
  single major release per year now.

Refs: https://github.com/nodejs/Release/issues/1187

PTAL @nodejs/build @nodejs/releasers

[nodejs-github-bot]

Review requested:

• @nodejs/actions
• @nodejs/releasers
• @nodejs/tsc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.49%. Comparing base (4096767) to head (83198bc).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66796      +/-   ##
==========================================
- Coverage   90.50%   90.49%   -0.01%     
==========================================
  Files         797      797              
  Lines      277652   277686      +34     
  Branches    53345    53344       -1     
==========================================
+ Hits       251282   251300      +18     
- Misses      16758    16792      +34     
+ Partials     9612     9594      -18     

see 27 files with indirect coverage changes

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