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

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 `` 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

Start at the block toolbar language selector and inspect how it currently uses the ToolbarGroup component, then check whether CustomSelectControl is available in supported WordPress versions. Done means users can type the first characters of a language and jump to the matching option, with the selector remaining usable through keyboard focus.

Written by the indexing model from the issue text.