refactor: change wrenCompileSource to return ObjClosure instead of ObjFiber and inline import execution

The compileInModule function now returns a closure directly instead of wrapping it in a fiber, simplifying the compilation pipeline and removing unnecessary fiber overhead for module imports. The wrenImportModule function is moved into wren_vm.c and rewritten to execute the imported module's closure immediately rather than returning a fiber for deferred execution. This change also adds a test for yielding from an imported module to verify correct behavior with the new execution model.

test/core/fiber/yield_from_import.wren

@@ -0,0 +1,14 @@
+var fiber = Fiber.new {
+  System.print("fiber 1")
+
+  import "yield_from_import_module"
+
+  System.print("fiber 2")
+}
+
+fiber.call()              // expect: fiber 1
+                          // expect: module 1
+System.print("main 1")    // expect: main 1
+fiber.call()              // expect: module 2
+                          // expect: fiber 2
+System.print("main 2")    // expect: main 2

test/core/fiber/yield_from_import_module.wren

@@ -0,0 +1,4 @@
+// nontest
+System.print("module 1")
+Fiber.yield()
+System.print("module 2")