Project panel horizontal scrollbar (#18513)

<img width="389" alt="image"
src="https://github.com/user-attachments/assets/c6718c6e-0fe1-40ed-b3db-7d576c4d98c8">


https://github.com/user-attachments/assets/734f1f52-70d9-4308-b1fc-36c7cfd4dd76

Closes https://github.com/zed-industries/zed/issues/7001
Closes https://github.com/zed-industries/zed/issues/4427
Part of https://github.com/zed-industries/zed/issues/15324
Part of https://github.com/zed-industries/zed/issues/14551

* Adjusts a `UniformList` to have a horizontal sizing behavior: the old
mode forced all items to have the size of the list exactly.
A new mode (with corresponding `ListItems` having `overflow_x` enabled)
lays out the uniform list elements with width of its widest element,
setting the same width to the list itself too.

* Using the new behavior, adds a new scrollbar into the project panel
and enhances its file name editor to scroll it during editing of long
file names

* Also restyles the scrollbar a bit, making it narrower and removing its
background

* Changes the project_panel.scrollbar.show settings to accept `null` and
be `null` by default, to inherit `editor`'s scrollbar settings. All
editor scrollbar settings are supported now.

Release Notes:

- Added a horizontal scrollbar to project panel
([#7001](https://github.com/zed-industries/zed/issues/7001))
([#4427](https://github.com/zed-industries/zed/issues/4427))

---------

Co-authored-by: Piotr Osiewicz <piotr@zed.dev>

crates/gpui/src/elements/list.rs

@@ -89,6 +89,16 @@ pub enum ListSizingBehavior {
     Auto,
 }
 
+/// The horizontal sizing behavior to apply during layout.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum ListHorizontalSizingBehavior {
+    /// List items' width can never exceed the width of the list.
+    #[default]
+    FitList,
+    /// List items' width may go over the width of the list, if any item is wider.
+    Unconstrained,
+}
+
 struct LayoutItemsResponse {
     max_item_width: Pixels,
     scroll_top: ListOffset,