feat: append ".wren" extension to module paths in readModule and update test imports

The readModule function now concatenates ".wren" to the module path when constructing the full file path, increasing the path buffer by 5 bytes. All test files have been updated to use bare module names (without ".wren") in import_ calls, and the expected error messages in unknown_module and unknown_variable tests are adjusted accordingly. A trailing blank line is also removed from wren_vm.c.

test/module/simple_import/simple_import.wren

@@ -1,4 +1,4 @@
-var Module = "module.wren".import_("Module")
+var Module = "module".import_("Module")
 // expect: ran module
 
 IO.print(Module) // expect: from module