Add subscribe method to state class

This makes the State class fully compatible with svelte stores

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "skooma",
-	"version": "1.6.1",
+	"version": "1.7.0",
 	"author": "darkwiiplayer",
 	"license": "Unlicense",
 	"main": "skooma.js",

state.js

@@ -160,6 +160,17 @@ export class State extends SimpleState {
 		return this.#target[prop]
 	}
 
+	subscribe(prop, callback) {
+		if (!callback) return this.subscribe("value", prop)
+
+		const controller = new AbortController()
+		this.addEventListener("change", ({final}) => {
+			if (final.has(prop)) return callback(final.get(prop))
+		}, {signal: controller.signal})
+		callback(this.value)
+		return () => controller.abort()
+	}
+
 	// Backwards compatibility
 	get proxy() { return this.values }
 }