feat: run example files as tests to catch regressions like #266

Add a new `run_example` function that invokes the existing test runner with an `example=True` flag, and walk the `example/` and `example/import-module/` directories to execute all example files as tests. The `example` flag suppresses output comparison against expected lines, allowing examples that produce arbitrary output (or none) to pass without failure. Also comment out `foreign static bar` and `foreign baz(string)` in `example/syntax.wren` to prevent compilation errors when running it as a test.

example/syntax.wren

@@ -90,8 +90,8 @@ class ReservedWords is SyntaxExample {
   }
 
   // `foreign`, `static`
-  foreign static bar
-  foreign baz(string)
+  // foreign static bar
+  // foreign baz(string)
   // (Remove lines above to make this file compile)
 }