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>

docs/src/configuring-zed.md

@@ -1954,7 +1954,7 @@ Run the `theme selector: toggle` action in the command palette to see a current
     "auto_reveal_entries": true,
     "auto_fold_dirs": true,
     "scrollbar": {
-      "show": "always"
+      "show": null
     }
   }
 }
@@ -2074,13 +2074,13 @@ Run the `theme selector: toggle` action in the command palette to see a current
 
 ### Scrollbar
 
-- Description: Scrollbar related settings. Possible values: "always", "never".
+- Description: Scrollbar related settings. Possible values: null, "auto", "system", "always", "never". Inherits editor settings when absent, see its description for more details.
 - Setting: `scrollbar`
 - Default:
 
 ```json
 "scrollbar": {
-    "show": "always"
+    "show": null
 }
 ```