Replace deprecated strong-mode options (#21)

Updates to match the new [analysis language
options](https://dart.dev/guides/language/analysis-options#enabling-additional-type-checks)
also used in
[`flutter/flutter`](https://github.com/flutter/flutter/blob/master/analysis_options.yaml).

Issue reference: dart-lang/sdk#50679

scroll_overlay/analysis_option.yaml

@@ -2,8 +2,9 @@
 #
 # Copied over from flutter/flutter repo.
 analyzer:
-  strong-mode:
-    implicit-dynamic: false
+  language:
+    strict-casts: true
+    strict-raw-types: true
   errors:
     # treat missing required parameters as a warning (not a hint)
     missing_required_param: warning