WB-586: Add changesets to automate the release process

Summary:

- Introduces `changesets` to the repository.
- Adds a new workflow to handle releases. This will be triggered after
  the "release coordinator" (one of WB's admins) has approved the
  release (by hitting the `Merge` button).
- Creates a new `release` script to handle releases using `changesets`
  instead of `lerna`. (keeping `lerna/publish` for backwards compatibility).

More info about the new release process here:
https://www.figma.com/file/xaYyzIuKLxqXAJVAl3y1S4/Wonder-Blocks-Web-Eng-process?node-id=50%3A400

NOTE: To include a regular PR in the release PR, the PR's author will
need to run `yarn changesets` to include the changes for each package
they have changed.

More info about using it on this part of the process here: https://www.figma.com/file/xaYyzIuKLxqXAJVAl3y1S4/Wonder-Blocks-Web-Eng-process?node-id=81%3A549

Issue: https://khanacademy.atlassian.net/browse/WB-586

Test plan:

Merge this PR and verify that the "Release PR" gets created after a
contributor lands a PR (merges it into `master`).

.changeset/README.md

@@ -0,0 +1,8 @@
+# Changesets
+
+Hello and welcome! This folder has been automatically generated by `@changesets/cli`, a build tool that works
+with multi-package repos, or single-package repos to help you version and publish your code. You can
+find the full documentation for it [in our repository](https://github.com/changesets/changesets)
+
+We have a quick list of common questions to get you started engaging with this project in
+[our documentation](https://github.com/changesets/changesets/blob/main/docs/common-questions.md)

.changeset/config.json

@@ -0,0 +1,10 @@
+{
+  "$schema": "https://unpkg.com/@changesets/config@1.6.2/schema.json",
+  "changelog": "@changesets/cli/changelog",
+  "commit": false,
+  "linked": [],
+  "access": "public",
+  "baseBranch": "master",
+  "updateInternalDependencies": "patch",
+  "ignore": []
+}

.github/workflows/release.yml

@@ -0,0 +1,49 @@
+name: Release
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  release:
+    name: Release
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repo
+        uses: actions/checkout@master
+        with:
+          # This makes Actions fetch all Git history so that Changesets can
+          # generate changelogs with the correct commits
+          fetch-depth: 0
+
+      - name: Setup Node.js 12.x
+        uses: actions/setup-node@master
+        with:
+          node-version: 12.x
+
+      - name: Install Dependencies
+        run: yarn
+
+      - name: Create Release Pull Request or Publish to npm
+        id: changesets
+        uses: changesets/action@master
+        with:
+          publish: yarn release
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          NPM_TOKEN: ${{ secrets.NPM_TOKEN }}
+
+      - name: Send a Slack notification if a publish happens
+      - uses: rtCamp/action-slack-notify@v2
+        if: steps.changesets.outputs.published == 'true'
+        env:
+          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK }}
+          SLACK_CHANNEL: wonder-blocks-eng
+          SLACK_MSG_AUTHOR: ${{ github.event.pull_request.user.login }}
+          SLACK_USERNAME: GithubGoose
+          SLACK_ICON_EMOJI: ":goose:"
+          SLACK_MESSAGE: "A new version of ${GITHUB_REPOSITORY} was published! [Release notes →](<https://github.com/Khan/wonder-blocks/releases/>)"
+          SLACK_TITLE: "New Wonder Blocks release!"
+          SLACK_FOOTER: Wonder Blocks Slack Notification
+          MSG_MINIMAL: true

package.json

@@ -21,7 +21,8 @@
     "jest": "jest --config config/jest/test.config.js",
     "lint:watch": "esw --watch --config ./eslint/eslintrc packages",
     "lint": "eslint --config .eslintrc.js packages",
-    "publish": "node utils/pre-publish-check.js && git diff --stat --exit-code HEAD && yarn build:cjs && yarn build:es6 && lerna publish",
+    "publish": "node utils/pre-publish-check.js && git diff --stat --exit-code HEAD && yarn build:all && lerna publish",
+    "release": "node utils/pre-publish-check.js && git diff --stat --exit-code HEAD && yarn build:all && changeset publish",
     "start:prod": "yarn run build:styleguidist && yarn run start:styleguidist --config styleguide.prod.config.js",
     "start:storybook": "start-storybook -s ./static -p 6061",
     "start:styleguidist": "styleguidist server",
@@ -41,6 +42,7 @@
     "@babel/plugin-transform-spread": "7.12.1",
     "@babel/preset-env": "^7.13.10",
     "@babel/preset-react": "^7.13.8",
+    "@changesets/cli": "^2.18.0",
     "@khanacademy/eslint-config": "0.0.5",
     "@storybook/addon-a11y": "^6.3.7",
     "@storybook/addon-essentials": "^6.3.7",