[meta] add exports field

If this turns out to be breaking, it means someone is using `require('is-date-object/index')` or requiring the test file, so that seems unlikely

Author: ljharb

Diff for: package.json

@@ -8,6 +8,10 @@
 	"description": "Is this value a JS Date object? This module works cross-realm/iframe, and despite ES6 @@toStringTag.",
 	"license": "MIT",
 	"main": "index.js",
+	"exports": {
+		".": "./index.js",
+		"./package.json": "./package.json"
+	},
 	"sideEffects": false,
 	"scripts": {
 		"prepack": "npmignore --auto --commentLines=autogenerated",