Feat: Typescript Support - added index.d.ts with full type definitions for BackgroundTimer (#20)

* Feat(types): add index.d.ts with full type definitions for BackgroundTimer

- Provides type safety and IntelliSense for BackgroundTimer usage
- Defines RNBackgroundTimer native module methods (`start`, `stop`, `setTimeout`, `clearTimeout`)
- Adds types for internal timer management (`callbacks`, `interval`, etc.)
- Enables compatibility with TypeScript projects using the library

* Format document

---------

Co-authored-by: boterop <boterop22@gmail.com>

Author: ZFLOC-Technologies

index.d.ts

@@ -0,0 +1,39 @@
+import { EmitterSubscription, TurboModule } from 'react-native';
+
+declare module '@boterop/react-native-background-timer' {
+	export interface TimerCallback {
+		callback: () => void;
+		interval: boolean;
+		timeout: number;
+	}
+
+	export interface RNBackgroundTimerModule extends TurboModule {
+		start: (delay?: number) => void;
+		stop: () => void;
+		setTimeout: (id: number, timeout: number) => void;
+		clearTimeout?: (id: number) => void;
+	}
+
+	export interface BackgroundTimerInterface {
+		uniqueId: number;
+		callbacks: Record<number, TimerCallback>;
+		backgroundTimer?: number;
+		backgroundListener?: EmitterSubscription;
+
+		start: (delay?: number) => void;
+		stop: () => void;
+
+		runBackgroundTimer: (callback: () => void, delay: number) => void;
+		backgroundClockMethod: (callback: () => void, delay: number) => void;
+		stopBackgroundTimer: () => void;
+
+		setTimeout: (callback: () => void, timeout: number) => number;
+		clearTimeout: (timeoutId: number) => void;
+
+		setInterval: (callback: () => void, timeout: number) => number;
+		clearInterval: (intervalId: number) => void;
+	}
+
+	const BackgroundTimer: BackgroundTimerInterface;
+	export default BackgroundTimer;
+}