editor: Add preserve scroll strategy for go to definition (#55036)

These changes attempt to expand on the work introduced by
https://github.com/zed-industries/zed/pull/54778 by introducing a new
`GoToDefinitionScrollStrategy::Preserve` variant that attempts to keep
the cursor at the same vertical offset within the viewport when
navigating to a definition.

Most of the machinery for this was already in place. To support cases
where the user's scroll position isn't snapped to an exact display row,
for example, after scrolling with the mmouse, `Autoscroll::TopRelative`
and `Autoscroll::BottomRelative` were updated from `usize` to
`ScrollOffset`, allowing fractional offsets.

When the cursor is offscreen at the moment the `editor: go to
definition` action is invoked, `Preserve` falls back to
`Autoscroll::center`, matching the existing default for
`go_to_definition_scroll_strategy`. This avoids attempting to preserve
an offset where the cursor isn't visible which would lead to the cursor
being offscreen when jumping to the definition.

Documentation has also been updated to reflect this new strategy value.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Relates to #52173

Release Notes:

- Added a new `preserve` option to `go_to_definition_scroll_strategy`
that keeps the cursor at the same vertical position within the viewport
when navigating to a definition

docs/src/reference/all-settings.md

@@ -2456,6 +2456,15 @@ Example:
 }
 ```
 
+4. Preserve the cursor's vertical position within the viewport, falling back to
+   `center` when the cursor is offscreen.
+
+```json [settings]
+{
+  "go_to_definition_scroll_strategy": "preserve"
+}
+```
+
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