[actions] Publish PR previews

[gibson042]

Fixes #30

• Rename .github/workflows/deploy.yml → .github/workflows/publish-main.yml.
• Replace .github/workflows/build.yml with .github/workflows/render-pr.yml, which uploads the result as an artifact.
• Introduce .github/workflows/publish-pr.yml, which runs on completion of render-pr.yml and publishes into gh-pages at pr/$PR (and adds a comment with that link).
• publish-pr.yml also adds a preview warning with code derived from ecma262 insert_snapshot_warning.js (see example at https://tc39.es/proposal-immutable-arraybuffer/pr/21/ ).
• Once this settles, I'll open a PR to align ecma262 preview warnings as well.

Preview Warning