core: add release step in CI to push packed tarballs to GH release

As part of #18 this is a test to try to push tarballs generated by
oclif to a Github Release.

The idea would for the release process to be the following:

- locally on a developer machine, bump the version, commit, tag and
  publish to npm:

  ```
  npm run publish -- --no-release-draft 2.0.1
  ```

- once the tag is pushed, the CI kicks in by doing the following:
  - Github release creation with upload of tarballs with this
    [github action](https://github.com/softprops/action-gh-release)

Author: paulRbr

.github/workflows/release.yml

@@ -0,0 +1,37 @@
+name: Release
+on:
+  workflow_run:
+    workflows: [ "Developer checks" ]
+    types: [ completed ]
+jobs:
+  release:
+    if: github.event.workflow_run.conclusion == 'success' && startsWith(github.ref, 'refs/tags/v')
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node_version: [ '15' ]
+    steps:
+      - uses: actions/checkout@v2
+      - name: Cache node modules
+        uses: actions/cache@v2
+        env:
+          cache-name: cache-node-modules
+        with:
+          # npm cache files are stored in `~/.npm` on Linux/macOS
+          path: ~/.npm
+          key: node-${{ matrix.node_version }}-build-${{ env.cache-name }}-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            node-${{ matrix.node_version }}-build-${{ env.cache-name }}-
+      - uses: actions/setup-node@v2
+        with:
+          node-version: ${{ matrix.node_version }}
+      - run: npm ci
+      - run: npm run pack
+      - name: Release
+        uses: softprops/action-gh-release@v1
+        with:
+          files: dist/**/*
+          draft: true
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_REPOSITORY: paulrbr/bump-node-cli