docs: add codecov badge

Author: himself65

README.md

@@ -2,6 +2,7 @@
 
 [![npm](https://img.shields.io/npm/v/@textea/json-viewer)](https://www.npmjs.com/package/@textea/json-viewer)
 [![npm](https://img.shields.io/npm/l/@textea/json-viewer)](https://github.com/TexteaInc/json-viewer/blob/main/LICENSE)
+[![codecov](https://codecov.io/gh/TexteaInc/json-viewer/branch/main/graph/badge.svg?token=r32mzVhrRl)](https://codecov.io/gh/TexteaInc/json-viewer)
 
 This is a React component for JSON viewer, but not only a JSON viewer.