Add builder for adding custom sources

src/entrypoint/providers.ts

@@ -0,0 +1,10 @@
+import { gatherAllEmbeds, gatherAllSources } from '@/providers/all';
+import { Embed, Sourcerer } from '@/providers/base';
+
+export function getBuiltinSources(): Sourcerer[] {
+  return gatherAllSources();
+}
+
+export function getBuiltinEmbeds(): Embed[] {
+  return gatherAllEmbeds();
+}