(modals): Add Modal<Context> interface.

src/types/index.d.ts

@@ -4,4 +4,5 @@ export * from './event.d';
 export * from './e621-types.d';
 export * from './handler.d';
 export * from './helper-types.d';
+export * from './modal.d';
 export * from './task.d';

src/types/modal.d.ts

@@ -0,0 +1,6 @@
+import { ModalSubmitInteraction } from 'discord.js';
+
+export interface Modal<Context> {
+  id: string;
+  execute(context: Context, interaction: ModalSubmitInteraction, ...args: any): Promise<void>;
+}