Add variable validation features to README and CLI; implement linting and strict mode for environment variables in executor and linter. Enhance VSCode extension documentation for improved user guidance.

README.md

@@ -33,6 +33,31 @@ Run in headless mode (default):
 node src/cli.js tests/example.test Chrome --headless
 ```
 
+## Variable Validation
+
+PlayWrong now provides comprehensive validation for environment variables:
+
+### Linting (Static Analysis)
+```bash
+node src/cli.js tests/example.test Chrome --lint
+```
+- **Common variables** like `$PASSWORD`, `$PASSWORDMAIL`, `$EMAIL`, `$USERNAME` show as info messages
+- **Undefined variables** that are not commonly defined show as warnings
+
+### Runtime Validation
+During execution, undefined variables:
+- Show prominent warnings with helpful instructions
+- Are used as literal text (e.g., `$UNDEFINED_VAR` stays as `$UNDEFINED_VAR`)
+- Can be treated as errors with `--strict-variables` flag
+
+### Strict Mode
+```bash
+node src/cli.js tests/example.test Chrome --strict-variables
+```
+- Treats undefined variables as errors
+- Stops execution immediately when an undefined variable is encountered
+- Useful for CI/CD pipelines to ensure all variables are properly set
+
 ## Test Language Syntax
 
 ### Profiles