editor: Fix panics in CursorPosition::update_position (#41237)

Fixes a regression introduced in
https://github.com/zed-industries/zed/pull/39857. As for the exact
reason this causes this issue I am not yet sure will investigate (as per
the todos in code)

Fixes ZED-23R

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Cargo.lock

@@ -14226,7 +14226,6 @@ dependencies = [
  "rand 0.9.2",
  "rayon",
  "regex",
- "smallvec",
  "sum_tree",
  "unicode-segmentation",
  "util",