Replace instanceof Object with typeof checks

Using `instanceof Object` is generally problematic, since it's not guaranteed to always do the right thing for all Objects.
(I stumbled upon this while working on another patch, when I noticed that the `outlineView` was broken with workers disabled.)

.eslintrc

@@ -166,6 +166,10 @@
     "no-nested-ternary": "error",
     "no-new-object": "error",
     "no-restricted-syntax": ["error",
+      {
+        "selector": "BinaryExpression[operator='instanceof'][right.name='Object']",
+        "message": "Use `typeof` rather than `instanceof Object`.",
+      },
       {
         "selector": "CallExpression[callee.name='assert'][arguments.length!=2]",
         "message": "`assert()` must always be invoked with two arguments.",