editor: drop use of react-monaco-editor

The upstream package is currently broken and doesn't actually save us
any work.

src/editor/editor.scss

@@ -26,6 +26,7 @@
     justify-content: flex-start;
 
     &-tablist {
+        flex: none;
         padding: bp.$pt-grid-size * 0.3;
         overflow-x: auto;
         @include pb.background-contrast(6%);
@@ -63,6 +64,11 @@
         flex: 1 1 auto;
     }
 
+    &-monaco {
+        width: 100%;
+        height: 100%;
+    }
+
     &-placeholder {
         pointer-events: none;
         width: max-content;