Rename flow-controller{.d -> }.ts

The file isn't included in the build artefacts otherwise, making the type
unavailable to consumers.

Fixes #594

Author: gustavohenke

lib/flow-control/flow-controller.d.ts lib/flow-control/flow-controller.tslib/flow-control/flow-controller.d.ts renamed to lib/flow-control/flow-controller.ts

@@ -1,4 +1,4 @@
-import { Command } from '../command.js';
+import type { Command } from '../command.js';
 
 /**
  * Interface for a class that controls and/or watches the behavior of commands.
@@ -7,5 +7,5 @@ import { Command } from '../command.js';
  * actually finish.
  */
 export interface FlowController {
-    handle: (commands: Command[]) => { commands: Command[]; onFinish?: () => void | Promise<void> };
+    handle(commands: Command[]): { commands: Command[]; onFinish?: () => void | Promise<void> };
 }