gpui: Add scroll_to_item_with_offset to UniformListScrollState (#35064)

Previously we had `ScrollStrategy::ToPosition(usize)` which lets you
define the offset where you want to scroll that item to. This is the
same as `ScrollStrategy::Top` but imagine some space reserved at the
top.

This PR removes `ScrollStrategy::ToPosition` in favor of
`scroll_to_item_with_offset` which is the method to do the same. The
reason to add this method is that now not just `ScrollStrategy::Top` but
`ScrollStrategy::Center` can also uses this offset to center the item in
the remaining unreserved space.

```rs
// Before
scroll_handle.scroll_to_item(index, ScrollStrategy::ToPosition(offset));

// After
scroll_handle.scroll_to_item_with_offset(index, ScrollStrategy::Top, offset);

// New! Centers item skipping first x items
scroll_handle.scroll_to_item_with_offset(index, ScrollStrategy::Center, offset);
```

This will be useful for follow up PR.

Release Notes:

- N/A

crates/project_panel/src/project_panel.rs

@@ -4207,10 +4207,7 @@ impl ProjectPanel {
                         this.marked_entries.clear();
                         if is_sticky {
                             if let Some((_, _, index)) = this.index_for_entry(entry_id, worktree_id) {
-                                let strategy = sticky_index
-                                    .map(ScrollStrategy::ToPosition)
-                                    .unwrap_or(ScrollStrategy::Top);
-                                this.scroll_handle.scroll_to_item(index, strategy);
+                                this.scroll_handle.scroll_to_item_with_offset(index, ScrollStrategy::Top, sticky_index.unwrap_or(0));
                                 cx.notify();
                                 // move down by 1px so that clicked item
                                 // don't count as sticky anymore