use rescript core option instead of belt

Author: rbertramResi

src/Playground.res

@@ -1485,8 +1485,8 @@ let make = (~versions: array<string>) => {
   let (keyMap, setKeyMap) = React.useState(() => {
     Dom.Storage2.localStorage
     ->Dom.Storage2.getItem("vimMode")
-    ->Belt.Option.map(CodeMirror.KeyMap.fromString)
-    ->Belt.Option.getWithDefault(CodeMirror.KeyMap.Default)
+    ->Option.map(CodeMirror.KeyMap.fromString)
+    ->Option.getOr(CodeMirror.KeyMap.Default)
   })
 
   React.useEffect1(() => {