add hook for mobile device check

src/lib/hooks/is-mobile.svelte.ts

@@ -0,0 +1,9 @@
+import { MediaQuery } from "svelte/reactivity";
+
+const DEFAULT_MOBILE_BREAKPOINT = 768;
+
+export class IsMobile extends MediaQuery {
+	constructor(breakpoint: number = DEFAULT_MOBILE_BREAKPOINT) {
+		super(`max-width: ${breakpoint - 1}px`);
+	}
+}