replace deprecated 'typings' field with standard 'types' in package.json

Author: silicon-selenion

modules/openapi-generator/src/main/resources/typescript-fetch/package.mustache

@@ -15,7 +15,7 @@
 {{/packageAsSourceOnlyLibrary}}
 {{^packageAsSourceOnlyLibrary}}
   "main": "./dist/index.js",
-  "typings": "./dist/index.d.ts",
+  "types": "./dist/index.d.ts",
 {{#supportsES6}}
   "module": "./dist/esm/index.js",
   "sideEffects": false,