tooling: notify team members if package.json/yarn has been updated (#2972)

This is a team tooling thing to let each other know if `yarn` needs to
be run upon checkout of a branch.

### Change Type

- [x] `internal` — Any other changes that don't affect the published
package[^2]

.husky/post-checkout

@@ -1,4 +1,15 @@
 #!/usr/bin/env sh
 . "$(dirname -- "$0")/_/husky.sh"
 
-rm -rf *.tsbuildinfo */*.tsbuildinfo */*/*.tsbuildinfo */*/*/*.tsbuildinfo */*/*/*/*.tsbuildinfo
+set -e
+
+rm -rf *.tsbuildinfo */*.tsbuildinfo */*/*.tsbuildinfo */*/*/*.tsbuildinfo */*/*/*/*.tsbuildinfo
+
+# This notifies the user if the yarn.lock file has changed.
+CHANGED=$(git diff "$1" "$2" --stat -- ./yarn.lock | wc -l)
+if (( CHANGED > 0 )); then
+    echo
+    echo "🚨 🚨 🚨 yarn.lock has changed! 🚨 🚨 🚨 "
+    echo "run 'yarn' to get the latest!"
+    echo
+fi

.husky/post-merge

@@ -0,0 +1,13 @@
+#!/usr/bin/env sh
+. "$(dirname -- "$0")/_/husky.sh"
+
+set -e
+
+# This notifies the user if the yarn.lock file has changed.
+CHANGED=$(git diff HEAD@{1} --stat -- ./yarn.lock | wc -l)
+if (( CHANGED > 0 )); then
+    echo
+    echo "🚨 🚨 🚨 yarn.lock has changed! 🚨 🚨 🚨 "
+    echo "run 'yarn' to get the latest!"
+    echo
+fi