Support React Native

@react-native-community/cli (used by expo and I think react native), parses package.json to determine package paths.

This is necessary to build react native projects. See react-native-community/cli#1168 for more details.

package.json

@@ -30,7 +30,8 @@
   "unpkg": "dist/d3-interpolate.min.js",
   "exports": {
     "umd": "./dist/d3-interpolate.min.js",
-    "default": "./src/index.js"
+    "default": "./src/index.js",
+    "./package.json": "./package.json"
   },
   "sideEffects": false,
   "dependencies": {