Combine the blocklyArrowTop and blocklyArrowBottom styling into the blocklyDropDownArrow styling

[BeksOmega]

1. Remove the blocklyArrowTop and blocklyArrowBottom CSS classes from being assigned to the dropdown div (source code). Instead, only assign blocklyDropDownArrow.
2. Combine the styling from blocklyArrowTop and blocklyArrowBottom in css.ts into the blocklyDropDownArrow styling. It should have a 1px solid border on all sides, and a 4px radius in the top left and bottom right.

This change should be made against the rc/v12.0.0 branch. This should be marked as a breaking change.

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

[BeksOmega]

I don't think this quite works because of how the elements get layered :/ The menu content doesn't quite cover the arrow, so it looks like a separate element instead of being part of the background. We could maybe add another separate background element? But I'm not sure if that's the right approach.

[mayank1365]

@BeksOmega please assign this issue to me