project panel: Add indent guides (#18260)

See #12673



https://github.com/user-attachments/assets/94079afc-a851-4206-9c9b-4fad3542334e



TODO:
- [x] Make active indent guides work for autofolded directories
- [x] Figure out which theme colors to use
- [x] Fix horizontal scrolling
- [x] Make indent guides easier to click
- [x] Fix selected background flashing when hovering over entry/indent
guide
- [x] Docs

Release Notes:

- Added indent guides to the project panel

crates/storybook/src/stories/indent_guides.rs

@@ -0,0 +1,83 @@
+use std::fmt::format;
+
+use gpui::{
+    colors, div, prelude::*, uniform_list, DefaultColor, DefaultThemeAppearance, Hsla, Render,
+    View, ViewContext, WindowContext,
+};
+use story::Story;
+use strum::IntoEnumIterator;
+use ui::{
+    h_flex, px, v_flex, AbsoluteLength, ActiveTheme, Color, DefiniteLength, Label, LabelCommon,
+};
+
+const LENGTH: usize = 100;
+
+pub struct IndentGuidesStory {
+    depths: Vec<usize>,
+}
+
+impl IndentGuidesStory {
+    pub fn view(cx: &mut WindowContext) -> View<Self> {
+        let mut depths = Vec::new();
+        depths.push(0);
+        depths.push(1);
+        depths.push(2);
+        for _ in 0..LENGTH - 6 {
+            depths.push(3);
+        }
+        depths.push(2);
+        depths.push(1);
+        depths.push(0);
+
+        cx.new_view(|_cx| Self { depths })
+    }
+}
+
+impl Render for IndentGuidesStory {
+    fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
+        Story::container()
+            .child(Story::title("Indent guides"))
+            .child(
+                v_flex().size_full().child(
+                    uniform_list(
+                        cx.view().clone(),
+                        "some-list",
+                        self.depths.len(),
+                        |this, range, cx| {
+                            this.depths
+                                .iter()
+                                .enumerate()
+                                .skip(range.start)
+                                .take(range.end - range.start)
+                                .map(|(i, depth)| {
+                                    div()
+                                        .pl(DefiniteLength::Absolute(AbsoluteLength::Pixels(px(
+                                            16. * (*depth as f32),
+                                        ))))
+                                        .child(Label::new(format!("Item {}", i)).color(Color::Info))
+                                })
+                                .collect()
+                        },
+                    )
+                    .with_sizing_behavior(gpui::ListSizingBehavior::Infer)
+                    .with_decoration(ui::indent_guides(
+                        cx.view().clone(),
+                        px(16.),
+                        ui::IndentGuideColors {
+                            default: Color::Info.color(cx),
+                            hovered: Color::Accent.color(cx),
+                            active: Color::Accent.color(cx),
+                        },
+                        |this, range, cx| {
+                            this.depths
+                                .iter()
+                                .skip(range.start)
+                                .take(range.end - range.start)
+                                .cloned()
+                                .collect()
+                        },
+                    )),
+                ),
+            )
+    }
+}