Add a `blocklyNotDeletable` CSS class to the block's root SVG

[BeksOmega]

To Fix

In the BlockSvg class, override the setDeletable method to call dom.addClass on the this.svgGroup_ to add a blocklyNotDeletable class when the block is not deletable, or dom.removeClass to remove the class when the block is deletable.

The PR to fix this should be made against the rc/v12.0.0 branch.

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

[Shreshthaaa]

Hey @BeksOmega, I would like to work on this issue.

[BeksOmega]

Sweetness, go for it @Shreshthaaa =)

[Shreshthaaa]

Hi @BeksOmega ,
I have submitted PR. Could you please review it?

I have written the code logically, but I'm unsure how to test it as I couldn't find the relevant test suites. Could you guide me on how to proceed with testing?
Thank you.