Filter the code block floating menu on keydown

Hello again,

This one's a bit more straightforward.

• You open the languages floating menu from a code block node
• You press a letter key
• The list changes the active option to the first one that starts with that letter (scrolls if necessary)
• If the key is pressed again, and multiple options start with that letter, the focus will move to the next one.

Again, if you're ok with the proposal, I can work on a PR for it.