Expose async-function argument type

We are exposing the async-function argument type for jsDoc
type declaration support. This means that we now could do:
"npm i -D @types/github-script@github:actions/github-script"
and the add:
"@param {import('@types/github-script').AsyncFunctionArguments}
AsyncFunctionArguments".

This could obviously be done in other ways too, like using
"@typed-actions/github-script" instead. But it seems better
to use the actual source repository instead of a third-party
library to import the type declaration.

package.json

@@ -5,9 +5,11 @@
   "author": "GitHub",
   "license": "MIT",
   "main": "dist/index.js",
+  "types": "types/async-function.d.ts",
   "private": true,
   "scripts": {
-    "build": "ncc build src/main.ts",
+    "build": "npm run build:types && ncc build src/main.ts",
+    "build:types": "tsc src/async-function.ts -t es5 --declaration --allowJs --emitDeclarationOnly --outDir types",
     "format:check": "prettier --check src __test__",
     "format:write": "prettier --write src __test__",
     "lint": "eslint src __test__",