fix: add flex: 1 to DrawerItem style (#8701)

# Summary 

In the [DrawerItem](https://github.com/react-navigation/react-navigation/blob/main/packages/drawer/src/views/DrawerItem.tsx), It's not possible to provide style for the [label wrapper](https://github.com/react-navigation/react-navigation/blob/main/packages/drawer/src/views/DrawerItem.tsx#L167), only for the label, so the label wrapper must at least be `flex: 1` in order to be more flexible for custom components, like in the pictures below.

In the below examples, providing `flex: 1` inside the custom component view is not enough, because the wrapper does not grow. This PR will fix it.

An alternative would be creating a new prop `labelStyleWrapper`, but I don't see any motivation for doing so, as this PR would fix it.

## Before

![before](https://user-images.githubusercontent.com/39778068/89813734-81b45a80-db18-11ea-8d53-0ddcfdbb04c1.jpeg)

## After

![after](https://user-images.githubusercontent.com/39778068/89813748-8711a500-db18-11ea-92b4-3db982c1eba5.jpeg)

packages/drawer/src/views/DrawerItem.tsx

@@ -205,6 +205,7 @@ const styles = StyleSheet.create({
   },
   label: {
     marginRight: 32,
+    flex: 1,
   },
   button: {
     display: 'flex',