Add automated publishing of weekly alpha releases to NPM

By default the ember-cli blueprint uses ember-try to download the `npm
pack` result of the most recent caanry build from Amazon S3 (this is
usually setup to run in that addon's CI configuration). Unfortunately,
some organizations have security restrictions **preventing** access to
the internet during CI jobs. They often have customized NPM mirrors that
do work (otherwise how would they ever build / test JS code), but block
access to the rest of the internet. Up until this change, those
organizations simply **could not** be early testers of our canary
builds.

This change adds a new CI job to auto-publish a new `-alpha.x` release
to NPM once a week (on Sundays). This will allow folks to leverage their
normal `npm install` infrastructure to be able to test canary Ember
builds once a week (though we could in theory decide to make more
publish). However, since the process is automated these alphas will not
include changelog entries. It is theoretically possible that we would
prefer to leverage this infrastructure for testing addons against
published tags (instead of using `ember-source-channel-url`), but that
would reduce our speed of identifiying regressions.

.github/workflows/alpha-releases.yml

@@ -0,0 +1,57 @@
+name: Alpha Releases
+
+on:
+  schedule:
+    - cron:  '0 7 * * 0' # weekly (sunday), 7am
+
+jobs:
+  test:
+    name: Basic Test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 10.x
+      - uses: actions/cache@v2
+        with:
+          path: '**/node_modules'
+          key: ci-modules-${{ hashFiles('**/yarn.lock') }}
+      - name: install dependencies
+        run: yarn install --frozen-lockfile --non-interactive
+      - name: build
+        env:
+          DISABLE_SOURCE_MAPS: true
+          BROCCOLI_ENV: production
+        run: yarn ember build
+      - name: test
+        env:
+          TEST_SUITE: each-package
+        run: yarn test
+
+  release:
+    name: Tag + Release
+    runs-on: ubuntu-latest
+    needs: [test]
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 10.x
+          registry-url: 'https://registry.npmjs.org'
+      - uses: actions/cache@v2
+        with:
+          path: '**/node_modules'
+          key: ci-modules-${{ hashFiles('**/yarn.lock') }}
+      - name: install dependencies
+        run: yarn install --frozen-lockfile --non-interactive
+      - name: tag the next alpha
+        run: npm version prerelease --preid alpha
+      - name: build for publish
+        run: node bin/build-for-publishing.js
+      - name: publish to npm
+        run: npm publish
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.NPM_AUTH_TOKEN }}
+      - name: push branch + tag
+        run: git push origin HEAD --follow-tags