migrate to expo-linear-gradient (#3420)

* replace packages

* rm useless alias from webpack config

* fix accidental double commit

* replace all usages

src/view/com/composer/Composer.tsx

@@ -11,8 +11,8 @@ import {
   TouchableOpacity,
   View,
 } from 'react-native'
-import LinearGradient from 'react-native-linear-gradient'
 import {useSafeAreaInsets} from 'react-native-safe-area-context'
+import LinearGradient from 'expo-linear-gradient'
 import {RichText} from '@atproto/api'
 import {FontAwesomeIcon} from '@fortawesome/react-native-fontawesome'
 import {msg, Trans} from '@lingui/macro'