Add a `blocklyTrashcanFlyout` CSS class to the flyout if it is used by the trashcan

[BeksOmega]

In the trashcan's init method, before inserting the result of this.flyout!.createDom into the DOM, call dom.addClass with the result of this.flyout!.createDom and blocklyTrashcanFlyout.

This change should be made against the rc/v12.0.0 branch.

To work on this issue, comment below asking to be assigned.

[ChaitanyaYeole02]

Hey. Can you assign the issue to me please, as discussed?

[BeksOmega]

Done! Let me know if you have an questions @ChaitanyaYeole02 =)

[siddharthbaleja7]

Hey.Can you please assign this issue to me?

[BeksOmega]

Sorry @siddharthbaleja7 This is already being worked on :/

[BeksOmega]

Closed by #8372