[structs] change package structutil to structs (#81)

* refactor package structutil to structs

* add structs package zh-CN docs

structs/struct_internal.go

@@ -0,0 +1,7 @@
+package structs
+
+import "fmt"
+
+func errInvalidStruct(v any) error {
+	return fmt.Errorf("invalid struct %v", v)
+}