.github/workflows: add action to update tapioca definitions

- Tapioca is a companion gem to Sorbet that generates RBI files for the
  Ruby gems in a project.
- Whenever Dependabot updates a gem, Tapioca has to regenerate that gem's
  RBI files so that Sorbet can accurately assess the state of the typing
  of a codebase.
- We also must regenerate Sorbet's view of
  [things defined at runtime](https://sorbet.org/docs/rbi#the-hidden-definition-rbi

):
  this is what it calls `hidden-definitions`.
- Obviously, this got tedious to do manually. So here's an Action that
  runs every three days and raises a PR if there are any changes.

Co-authored-by: Vidushee Amoli <vidushee.amoli@gmail.com>
Co-authored-by: Issy Long <me@issyl0.co.uk>
Co-authored-by: Jonathan Chang <jchang641@gmail.com>
Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>

.github/workflows/tapioca-update.yml

+name: Tapioca Update
+on:
+  push:
+    paths:
+      - .github/workflows/tapioca-update.yml
+  schedule:
+    - cron: '0 0 */3 * *' # every three days
+env:
+  HOMEBREW_SORBET: 1
+jobs:
+  update-tapioca:
+    if: github.repository == 'Homebrew/brew'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Homebrew
+        id: set-up-homebrew
+        uses: Homebrew/actions/setup-homebrew@master
+
+      - name: Configure Git user
+        uses: Homebrew/actions/git-user-config@master
+        with:
+          username: BrewTestBot
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.6.3
+
+      - name: Update Tapioca Definitions
+        run: |
+          cd "$GITHUB_WORKSPACE/Library/Homebrew"
+          bundle install
+          bundle exec tapioca sync --exclude json
+          bundle exec srb rbi hidden-definitions
+          if ! git diff --exit-code -- sorbet; then
+            BRANCH="tapioca-update"
+            if ! git ls-remote --exit-code --heads origin "$BRANCH"; then
+              git checkout -b "$BRANCH"
+              git commit -am "sorbet: Update gem RBI files using Tapioca
+
+              Autogenerated by [a scheduled GitHub Action](https://github.com/Homebrew/brew/actions?query=workflow%3A%22Tapioca+Update%22)."
+            fi
+          fi
+
+      - name: Push commits
+        uses: Homebrew/actions/git-try-push@master
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          branch: tapioca-update
+          directory: Library/Homebrew/
+
+      - name: Open a PR
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          cd "$GITHUB_WORKSPACE/Library/Homebrew"
+          hub pull-request -m "$(git log -1 --format='%s')"