Add type-to-search to the block toolbar language selector

[yscik]

In the block toolbar language selector, allow jumping to an option by typing it's first characters, just like regular select boxes work.

It can be a bit messy to do using the current <ToolbarGroup> component, needing keyboard handling and focus implementation. CustomSelectControl with CSS adjustment could work out better, if it's available in supported Wordpress versions.

Additional context

#172