refactor: convert postman case

hrp/pkg/convert/to_json.go

@@ -0,0 +1,13 @@
+package convert
+
+import "github.com/httprunner/httprunner/v4/hrp/internal/builtin"
+
+// convert TCase to JSON case
+func (c *TCaseConverter) toJSON() (string, error) {
+	jsonPath := c.genOutputPath(suffixJSON)
+	err := builtin.Dump2JSON(c.tCase, jsonPath)
+	if err != nil {
+		return "", err
+	}
+	return jsonPath, nil
+}