Add commit hook for verifying that we transpiled the TypeScript code

.githooks/pre-commit

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+# This commit hook checks whether we ran `npm run build` when committed TypeScript files.
+# For GitHub actions to work, we need to check the compiled JavaScript into VCS.
+#
+# This script can yield false positives in cases where you only make stylistic changes to the TypeScript code that don't result in changes to the compiled JavaScript code.
+# It is your responsibility as a developer to then commit the changes with `git commit --no-verify` and simply skip this commit hook.
+
+TS_FILES=$(git diff --staged --name-only | grep -c '.ts')
+DIST_MODIFIED=$(git diff --staged --name-only | grep -c dist/index.js)
+
+if [ $TS_FILES -gt 0 ] && [ $DIST_MODIFIED -eq 0 ] ; then
+    echo "You modified TypeScript files but apparently did not run 'npm run build'".
+    exit 1;
+fi