feat: add version-resolver for PR-based version bumps (#12)

Switch from $NEXT_PATCH_VERSION to $RESOLVED_VERSION with version-resolver
config. This enables major/minor/patch version bumps via PR labels.

Author: eddmann

.github/release-drafter.yml

@@ -1,6 +1,19 @@
-name-template: $NEXT_PATCH_VERSION
-tag-template: $NEXT_PATCH_VERSION
+name-template: $RESOLVED_VERSION
+tag-template: $RESOLVED_VERSION
 commitish: draft-release
+
+version-resolver:
+  major:
+    labels:
+      - 'major'
+  minor:
+    labels:
+      - 'minor'
+  patch:
+    labels:
+      - 'patch'
+  default: patch
+
 template: |
   ## Changes