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.

types/async-function.d.ts

@@ -0,0 +1,20 @@
+/// <reference types="node" />
+import * as core from '@actions/core';
+import * as exec from '@actions/exec';
+import { Context } from '@actions/github/lib/context';
+import { GitHub } from '@actions/github/lib/utils';
+import * as glob from '@actions/glob';
+import * as io from '@actions/io';
+import fetch from 'node-fetch';
+export declare type AsyncFunctionArguments = {
+    context: Context;
+    core: typeof core;
+    github: InstanceType<typeof GitHub>;
+    exec: typeof exec;
+    glob: typeof glob;
+    io: typeof io;
+    fetch: typeof fetch;
+    require: NodeRequire;
+    __original_require__: NodeRequire;
+};
+export declare function callAsyncFunction<T>(args: AsyncFunctionArguments, source: string): Promise<T>;