Make custom HTML blocks resizable

[Pikamander2]

I don't always need to see every line of code in a custom HTML block, especially when I'm editing a completely different element. Sometimes I just want to minimize it or limit it to a certain height.

Please consider adding a stretch handle or minimize button.

[mtias]

We might also consider minimizing by default when the block is not focused.

[karmatosed]

I would 👍 the ides of resizing and also minimising by default.

[chrisvanpatten]

I'm happy to take a crack at this with the new ResizableBox component once #10396 lands.

[karmatosed]

Thanks @chrisvanpatten!