Use current state to compute format edit range

Previously, the server computed the "whole document" edit range from the
formatted document, which produced weird results if the formatter
removed lines, etc (e.g. `{\n}` --> `{}`).

Now, we use the current state to compute the edit range, as intended.

Fixed: 103606
Change-Id: I99b373f73667ce1c4284f98dc56a159d55820c80
Reviewed-on: https://fuchsia-review.googlesource.com/c/fidl-misc/+/707825
Reviewed-by: Alex Zaslavsky <azaslavsky@google.com>
Reviewed-by: Solly Ross <sollyross@google.com>