HTML Block: Consider allowing users to convert between HTML and Code blocks

[kjellr]

As originally raised in #8435, it might make sense to allow users to use the block switcher to convert between a Custom HTML block and a Code block.

This would potentially benefit use cases where someone has entered code into the "Code" block, thinking that code will run on the front end. Or, for cases where a user has chosen the HTML block to write some example HTML code, thinking it'll render as code on the front end.

[draganescu]

I think this is a good ideae, added #21779 as a quick try, but it doesn't seem to cover all cases.