Upgrade to Yarn Berry

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

Closes: #1734

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

@@ -30,12 +30,13 @@ jobs:
           node-version: ${{ matrix.node-version }}
           cache: 'yarn'
 
+      # TODO: this is no longer true:
       # Command `yarn install --frozen-lockfile` doesn't always fail if `yarn.lock` is modified
       # (see: https://github.com/yarnpkg/yarn/issues/5840). Instead, we are installing the
       # dependencies as usual and checking that nothing changed.
       - name: Install Yarn dependencies
         run: |
-          yarn install
+          yarn install --immutable
           git diff --exit-code
 
       # We are purposefully expecting zero warnings in the output to make sure flowtests are

.github/workflows/npm-publisher.yml

@@ -26,5 +26,5 @@ jobs:
 
       - name: publish NPM packages to the registry
         run: |
-          yarn install --frozen-lockfile
+          yarn install --immutable
           yarn monorepo-babel-node scripts/publishNPMPackages.js

.github/workflows/shipit.yml

@@ -25,7 +25,7 @@ jobs:
           cache: 'yarn'
 
       - name: Install Yarn dependencies
-        run: yarn install --frozen-lockfile
+        run: yarn install --immutable
 
       - name: Prepare Shipit
         run: |

.gitignore

@@ -12,6 +12,12 @@ 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