Upgrade to Yarn Berry

See: https://yarnpkg.com/getting-started/migration

Closes: #1734

.github/workflows/continuous-integration-javascript.yml

@@ -34,7 +34,7 @@ jobs:
         # Command `yarn install --frozen-lockfile` doesn't always fail if `yarn.lock` is modified (https://github.com/yarnpkg/yarn/issues/5840).
         # Instead, we are installing the dependencies as usual and checking that nothing changed.
         run: |
-          yarn install
+          yarn install --immutable --immutable-cache
           git diff --exit-code
 
       - name: Run Flow checks

.github/workflows/npm-publisher.yml

@@ -24,5 +24,5 @@ jobs:
           node-version: 14.x
       - name: publish NPM packages to the registry
         run: |
-          yarn install --frozen-lockfile
+          yarn install --immutable --immutable-cache
           yarn monorepo-babel-node scripts/publishNPMPackages.js

.github/workflows/shipit.yml

@@ -27,5 +27,5 @@ jobs:
           echo "${{ secrets.SSH_SHIPIT_PRIVATE_KEY }}" | tr -d '\r' > ~/.ssh/id_rsa
           chmod 600 ~/.ssh/id_rsa
           ssh-keyscan -H 'github.com' >> ~/.ssh/known_hosts
-          yarn install --frozen-lockfile
+          yarn install --immutable --immutable-cache
           yarn monorepo-babel-node src/monorepo-shipit/bin/shipit.js

.gitignore

@@ -14,6 +14,12 @@ node_modules/
 
 .DS_Store
 .env
-.flow.saved_state
-.flow.saved_state_file_changes
-yarn-error.log
+
+# Yarn Berry with "zero installs":
+.yarn/*
+#!.yarn/cache - TODO: commit
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions