add AddShape api

id.go

@@ -0,0 +1,10 @@
+package docx
+
+func (f *Docx) IncreaseID(name string) (n uintptr) {
+	f.slowIDsMu.Lock()
+	n, _ = f.slowIDs[name] //nolint: go-staticcheck
+	n++
+	f.slowIDs[name] = n
+	f.slowIDsMu.Unlock()
+	return
+}