[meta] add exports

Hopefully this technically-breaking change won't break anyone in practice.

Specifiers this continues to allow:
 - is-async-function
 - is-async-function/package.json

Specifiers this no longer allows:
 - is-async-function/
 - is-async-function/index
 - is-async-function/index.d
 - is-async-function/index.d.ts
 - is-async-function/index.js
 - is-async-function/package
 - is-async-function/test
 - is-async-function/test/
 - is-async-function/test/index
 - is-async-function/test/index.js
 - is-async-function/test/uglified
 - is-async-function/test/uglified.js
 - is-async-function/tsconfig
 - is-async-function/tsconfig.json

Author: ljharb

package.json

@@ -3,6 +3,10 @@
 	"version": "2.1.0",
 	"description": "Determine if a function is a native async function.",
 	"main": "index.js",
+	"exports": {
+		".": "./index.js",
+		"./package.json": "./package.json"
+	},
 	"scripts": {
 		"prepack": "npmignore --auto --commentLines=autogenerated",
 		"prepublishOnly": "safe-publish-latest",