Pass a selection range in CompletionParams for a completion request

[metalogical]

related to #377

It would be very useful for the completion request interface to support sending selection ranges instead of being limited to a cursor position.

Examples of scenarios where this would be useful:

• the user selects an expression and wants to replace it
• the user inserts a snippet (perhaps from a previous completion), and wants completions for a selected placeholder

This is a serious deficiency, because there is no way for the server to infer the intent of the user to replace the selection with a completion. That intent must inform the returned completions set.

A possible further extension of this is to support multi-cursor and multi-selection completions.