Add yarn.lock to .gitignore to prevent accidental yarn usage

This project uses npm as the package manager (package-lock.json).

Added yarn.lock to .gitignore to prevent it from being accidentally
created and committed if someone runs 'yarn install' by mistake.

Note: If yarn install is run, it will show warnings about package-lock.json
and the 'vscode' engine. These warnings are unavoidable in an npm-based
project and should be ignored. Use 'npm install' instead.

Author: dduugg

.gitignore

@@ -72,6 +72,9 @@ web_modules/
 # Yarn Integrity file
 .yarn-integrity
 
+# Yarn lock file (project uses npm, not yarn)
+yarn.lock
+
 # dotenv environment variable files
 .env
 .env.development.local