Use copilot's `Completion::{range,text}` to determine suggestion

Previously, we were using display text, but this isn't always correct. Now,
we just attempt to determine what text Copilot wants to insert by finding
a prefix and suffix in the existing text with the suggested text.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
6e43e77c3fAntonio Scandurra committed on 3/31/2023, 3:08:41 PMparent9b16277
2 files changedLine totals unavailable