feat: build initial prototype (#1)

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- [x] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
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- [x] I have pinned the dependencies to a specific patch version.

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- [x] I have run the linter and resolved any errors.
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### Tests

- [x] My contribution adds new code, and I have added tests to cover it.
- [ ] My contribution modifies existing code, and I have updated the tests to reflect these changes.
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Reviewed-on: #1
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src/utils/checkEntitlement.ts

@@ -0,0 +1,31 @@
+/**
+ * @copyright nhcarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+import { entitledGuilds } from "../config/entitlements.js";
+import type { Client, Guild } from "discord.js";
+
+/**
+ * Checks if a guild has subscribed.
+ * @param chibika - Chibika's Discord instance.
+ * @param guild - The guild to check.
+ * @returns A boolean indicating whether the guild has an active subscription.
+ */
+const checkGuildEntitlement = async(
+  chibika: Client,
+  guild: Guild,
+): Promise<boolean> => {
+  if (entitledGuilds.includes(guild.id)) {
+    return true;
+  }
+  const entitlements = await chibika.application?.entitlements.fetch({
+    excludeDeleted: true,
+    excludeEnded:   true,
+    guild:          guild,
+  });
+  return Boolean(entitlements && entitlements.size > 0);
+};
+
+export { checkGuildEntitlement };