fix: include "main" and "types" fields to support legacy applications

Without these fields, applications which use certain legacy TypeScript
settings (such as `"moduleResolution": "node"`) were not able to detect
the type definitions for `@optum/react-core`.

Author: nthurow

package.json

@@ -12,6 +12,8 @@
   "files": [
     "build/**"
   ],
+  "main": "./build/cjs/index.js",
+  "types": "./build/types/cjs/index.d.ts",
   "devDependencies": {
     "@babel/preset-env": "^7.12.11",
     "@babel/preset-typescript": "^7.12.10",