[Finishes #80927658] Allow variable create/edit/delete to be disabled on a per-level basis

This fix for #80927658 was a little more complex -- getUserBlocks() won't return blocks that are not editable.  I added a config option so editing of variables can be disabled, while keeping the block marked as editable.
Can't we just check Blockly.disableVariableEditing and flip the order here rather than adding an extra level of indirection?