implemented plus button on budget screen and routing for the plus button

app/(tabs)/budget/_layout.tsx

@@ -0,0 +1,14 @@
+import { Stack } from "expo-router";
+import { useTheme } from "../../contexts/ThemeContext";
+
+export default function _Layout() {
+    const { colors } = useTheme();
+    return (
+        <Stack initialRouteName="index" screenOptions={{
+            headerShown: false
+          }}>
+            <Stack.Screen name="index"/>
+            <Stack.Screen name="addCategory"/>
+        </Stack>
+    );
+}