Fix and clarify workflow logic, add branch versioning

- `pr.yml` should've been deleted a long time ago. Its functionality
  was already elsewhere, and that's since been folded into `test.yml`.
- I added automatic `release-v*` and `main-v*` versioning.
- Renamed `merge.yml` to `push-release.yml` and `push-main.yml` to
  `pr-create-release.yml` so it's clearer what they're actually
  listening to. I also modified their display names.

scripts/set-versioned-branch.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+base="$1"
+
+if [[ -z "$base" ]]; then
+    echo '::error::Base branch is missing. Invoke as `bash scripts/set-versioned-branch.sh BASE' >&2
+    exit 1
+fi
+
+major=$(node -pe 'require("./package.json").version.replace(/\..*$/,"")')
+# Can't do a force push due to branch protection rules.
+git checkout "${base}"
+git checkout -B "${base}-v${major}"
+git push origin "${base}-v${major}"