feat: add effect cleanups, add docs (#2)

* docs: update readme

* feat: add cleanup function to effects

* refactor: move peek function to store module

* test: add nested atom check

* docs: add doc comments

* fix: `effect()` type

* refactor: remove divider comment

src/effect.luau

@@ -1,10 +1,26 @@
 local store = require(script.Parent.store)
 
-local function effect(molecule: () -> nil): () -> ()
-	local captured = store.capture(molecule)
+type Cleanup = () -> ()
+
+--[=[
+	Runs the given callback immediately and whenever any atom it depends on
+	changes. Returns a cleanup function that unsubscribes the callback.
+	
+	Note that dependencies are only evaluated once when the effect is created,
+	and conditional dependencies may not work as expected.
+	
+	@param callback The function to run.
+	@return A function that unsubscribes the callback.
+]=]
+local function effect(callback: () -> any): () -> ()
+	local captured, cleanup = store.capture(callback)
 
 	local function listener()
-		molecule()
+		if cleanup then
+			cleanup()
+		end
+
+		cleanup = callback()
 	end
 
 	for atom in captured do
@@ -15,7 +31,11 @@ local function effect(molecule: () -> nil): () -> ()
 		for atom in captured do
 			store.listeners[atom][listener] = nil
 		end
+
+		if cleanup then
+			cleanup()
+		end
 	end
 end
 
-return effect
+return effect :: (callback: (() -> ()) | (() -> Cleanup)) -> Cleanup