compile source to UMD as well

Co-Authored-By: Keith Cirkel <keithamus@users.noreply.github.com>

package.json

@@ -2,7 +2,7 @@
   "name": "@github/textarea-autosize",
   "description": "Autosizes textarea to size of it's contents.",
   "version": "0.1.3",
-  "main": "dist/index.js",
+  "main": "dist/umd/index.js",
   "module": "dist/index.js",
   "types": "dist/index.d.ts",
   "license": "MIT",
@@ -14,7 +14,7 @@
     "clean": "rm -rf dist",
     "lint": "github-lint",
     "prebuild": "npm run clean && npm run lint && mkdir dist",
-    "build": "tsc",
+    "build": "tsc --outDir dist/umd/ --module umd && tsc",
     "test": "echo",
     "pretest": "npm run build",
     "prepublishOnly": "npm run build",