feat: add logical import support with wren_modules directory resolution and path type classification

Introduce a path type classification system (absolute, relative, simple) to distinguish logical imports from relative ones. Implement automatic discovery of a "wren_modules" directory by walking up from the root directory. Refactor the CLI to return interpret results and exit codes instead of exiting directly. Extract cross-platform stat macros into a shared header. Update test API calls to use explicit relative paths.

test/language/module/use_nearest_modules_dir/a/b/use_nearest_modules_dir.wren

@@ -0,0 +1,3 @@
+// Stops as soon as it finds a wren_modules directory, regardless of whether or
+// not it contains the desired module.
+import "foo" // expect runtime error: Could not load module 'foo'.