[Artifacts] Prepare for v2.0.0 of @actions/artifact (#1479)

* Prepare for v2.0.0 of @actions/artifact

* Run prettier

* temporary disable unused vars

src/artifact.ts

@@ -0,0 +1,12 @@
+import {ArtifactClient, Client} from './internal/client'
+import {UploadOptions} from './internal/upload/upload-options'
+import {UploadResponse} from './internal/upload/upload-response'
+
+/**
+ * Exported functionality that we want to expose for any users of @actions/artifact
+ */
+export {ArtifactClient, UploadOptions, UploadResponse}
+
+export function create(): ArtifactClient {
+  return Client.create()
+}