Accepting an LSP completion after typing more characters than the request saw (`System.out.` requested, then `Print` typed while the cached list stayed open) inserted the item after the typed text: `System.out.Printprint()`. The cached edit range started exactly at the request caret and was mapped forward past the newly typed characters. The range start now stays at the request caret, so the typed prefix is replaced, whatever its casing. Enter (insert) and Tab (replace) keep their behaviour for the text after the caret; this applies to every language server.
Fixes#316