langserver's autocompletion does not respect user's Insert Mode setting

[jwong-beep]

In VSCode, one is able to select whether completion suggestions are inserted at the cursor or replace some text on the line range determined by the language server. Since #3244, every completion after a : is returned as a text replacement of the entire target, including the :.

The correct implementation should return an InsertReplaceEdit, which includes both the ranges for insertion and replacement for the client to choose.

blocked by #3478