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/logical_dir/wren_modules/bar/within_bar.wren

@@ -0,0 +1,5 @@
+// nontest
+var Bar = "from bar"
+System.print("ran bar module")
+
+import "foo/within_foo"