AK: Have JsonArray::set() change values instead of inserting values

Resolves #18618.

8134dcc changed `JsonArray::set()` to insert elements at an index
instead of changing existing elements in-place. Since no behavior
such as `Vector::try_at()` exists yet, it returns nothing.

AK/JsonArray.h

@@ -66,7 +66,7 @@ public:
 
     void clear() { m_values.clear(); }
     ErrorOr<void> append(JsonValue value) { return m_values.try_append(move(value)); }
-    ErrorOr<void> set(size_t index, JsonValue value) { return m_values.try_insert(index, move(value)); }
+    void set(size_t index, JsonValue value) { m_values.at(index) = move(value); }
 
     template<typename Builder>
     typename Builder::OutputType serialized() const;

Userland/Libraries/LibGUI/JsonArrayModel.cpp

@@ -97,7 +97,7 @@ ErrorOr<void> JsonArrayModel::set(int row, Vector<JsonValue>&& fields)
         obj.set(field_spec.json_field_name, move(fields.at(i)));
     }
 
-    TRY(m_array.set(row, move(obj)));
+    m_array.set(row, move(obj));
     did_update();
 
     return {};