Add new mapper

mapper.go

@@ -86,3 +86,27 @@ func ExtractSelect(prop notionapi.Property) string {
 	}
 	return ""
 }
+
+func ExtractFormula(prop notionapi.Property) notionapi.Formula {
+	if form, ok := prop.(*notionapi.FormulaProperty); ok {
+		return form.Formula
+	}
+
+	return notionapi.Formula{}
+}
+
+func ExtractFormulaText(prop notionapi.Property) string {
+	return ExtractFormula(prop).String
+}
+
+func ExtractFormulaBool(prop notionapi.Property) bool {
+	return ExtractFormula(prop).Boolean
+}
+
+func ExtractFormulaNumber(prop notionapi.Property) float64 {
+	return ExtractFormula(prop).Number
+}
+
+func ExtractFormulaDate(prop notionapi.Property) notionapi.DateObject {
+	return *ExtractFormula(prop).Date
+}