chore: add auto-format npm script (#214)

This change should have no user impact.

`npm run format` will run Prettier on all files.

package.json

@@ -13,6 +13,7 @@
     "build:tsc": "tsc",
     "build:copy": "cpy 'intermediate/generated/**' 'intermediate/src/**' dist/",
     "build:doc": "typedoc src/index.ts --tsconfig tsconfig.typedoc.json --plugin typedoc-plugin-markdown",
+    "format": "prettier --write .",
     "lint": "prettier --check .",
     "prepack": "npm run build",
     "pretest": "npm run lint",