Widgets editor: Toolbar is not left-aligned with the selected block

[noisysocks]

Description

The block toolbar in the Widgets screen is not left-aligned with the selected block.

Step-by-step reproduction instructions

1. Go to Appearance → Widgets.
2. Select any top level block in any widget area.

Expected behaviour

Block toolbar should be left-aligned with the selected block.

Actual behaviour

Block toolbar is indented.

Screenshots or screen recording (optional)

[noisysocks]

This happens because the selected block is a child of the Widget Area block. This means that BlockContextualToolbar applies the has-parent class which indents the toolbar. This is so that there is space for BlockParentsSelector to appear. But BlockParentsSelector never shows because Widget Area sets __experimentalParentSelector to false.

We should not apply the has-parents class if __experimentalParentSelector is set to false.