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.

src/async-function.ts

@@ -8,7 +8,7 @@ import fetch from 'node-fetch'
 
 const AsyncFunction = Object.getPrototypeOf(async () => null).constructor
 
-type AsyncFunctionArguments = {
+export declare type AsyncFunctionArguments = {
   context: Context
   core: typeof core
   github: InstanceType<typeof GitHub>