[Toolkits] Allow overwrite of bracket when inserting functions from API toolkit

[microbit-matt-hillsdon]

Autocompleting to a function uses the same code in CodeMirror that's normally used for inserting a bracket so you get a bracket pair. The closing bracket can be overwritten. Ideally we'd reuse this same code when performing API drops.

I suspect this will be hard and/or not worth it so I've split it out so we can discuss priority.

[microbit-matt-hillsdon]

Historical note: the edits above are because the section now known as API was formerly called Reference (!)