fix(package): add "/src" to files to fix source map warning

Relates to remarkablemark/html-react-parser#1097

```
WARNING in ./node_modules/style-to-object/cjs/index.js
Module Warning (from ./node_modules/react-scripts/node_modules/source-map-loader/dist/cjs.js):
Failed to parse source map from 'node_modules/style-to-object/src/index.ts' file: Error: ENOENT: no such file or directory, open 'node_modules/style-to-object/src/index.ts'
```

package.json

@@ -70,7 +70,8 @@
   "files": [
     "/cjs",
     "/dist",
-    "/esm"
+    "/esm",
+    "/src"
   ],
   "license": "MIT"
 }