Project panel: Prevent scrollbar size from scaling with rem size (#14167)

The underlying container had width of 0.75 rem, which was equal to 12px
at default ui_font_size. However, with larger values of ui_font_size the
scrollbar would drift towards the center of a project panel, as the
scrollbar itself has a fixed width of 12 pixels. This commit moves
towards using a fixed width of 12px for scrollbar container. The
alternative was to make the scrollbar scale with ui_font_size, but that
isn't what the Editor scrollbar does, so I decided against it.



Release Notes:

- Fixed position of scrollbar in project panel with non-default
`ui_font_size` values.

crates/project_panel/src/project_panel.rs

@@ -2293,7 +2293,7 @@ impl ProjectPanel {
                 .right_0()
                 .top_0()
                 .bottom_0()
-                .w_3()
+                .w(px(12.))
                 .cursor_default()
                 .child(ProjectPanelScrollbar::new(
                     percentage as f32..end_offset as f32,