Rename the `blocklyTreeSelected` CSS class to `blocklyToolboxSelected`

[BeksOmega]

Rename any references to blocklyTreeSelected to blocklyToolboxSelected.

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.

[Harsh-svg988]

@BeksOmega can you assign this to me

[BeksOmega]

Hiya @Harsh-svg988 Go for it =)

[Adityajaiswal03]

hey @Harsh-svg988 can you please assign this issue to me

[omwagh28]

@Harsh-svg988 I haved solved this issue could you assign it to me if you aren't working on this please

[omwagh28]

@BeksOmega I collaborated with @Harsh-svg988 and solved this he unassigned himself so that i can solve this issue
I have raised a PR for it