fix(build): separate node, browser, and webpack exports

Author: cwillisf

package.json

@@ -10,8 +10,11 @@
     "url": "https://github.com/scratchfoundation/scratch-vm.git",
     "sha": "e7962b101d669f70146dc9cc455a7925523fb8d2"
   },
-  "main": "./dist/node/scratch-vm.js",
-  "browser": "./src/index.js",
+  "exports": {
+      "browser": "./dist/web/scratch-vm.js",
+      "node": "./dist/node/scratch-vm.js",
+      "webpack": "./src/index.js"
+  },
   "scripts": {
     "build": "npm run docs && webpack --progress",
     "coverage": "tap ./test/{unit,integration}/*.js --coverage --coverage-report=lcov",