Add placeholder git panel (#21894)

Adds a simple git placeholder panel for us to iterate from. Also
includes a number of assets from the git prototyping branch that we will
use.

Note: This panel is staff flagged for now.

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -200,6 +200,7 @@ pub enum IconName {
     GenericRestore,
     Github,
     Globe,
+    GitBranch,
     Hash,
     HistoryRerun,
     Indicator,
@@ -224,6 +225,8 @@ pub enum IconName {
     Option,
     PageDown,
     PageUp,
+    PanelLeft,
+    PanelRight,
     Pencil,
     Person,
     PhoneIncoming,
@@ -267,6 +270,9 @@ pub enum IconName {
     SparkleFilled,
     Spinner,
     Split,
+    SquareDot,
+    SquareMinus,
+    SquarePlus,
     Star,
     StarFilled,
     Stop,
@@ -497,7 +503,7 @@ impl RenderOnce for IconDecoration {
 }
 
 impl ComponentPreview for IconDecoration {
-    fn examples(cx: &WindowContext) -> Vec<ComponentExampleGroup<Self>> {
+    fn examples(cx: &mut WindowContext) -> Vec<ComponentExampleGroup<Self>> {
         let all_kinds = IconDecorationKind::iter().collect::<Vec<_>>();
 
         let examples = all_kinds
@@ -539,7 +545,7 @@ impl RenderOnce for DecoratedIcon {
 }
 
 impl ComponentPreview for DecoratedIcon {
-    fn examples(cx: &WindowContext) -> Vec<ComponentExampleGroup<Self>> {
+    fn examples(cx: &mut WindowContext) -> Vec<ComponentExampleGroup<Self>> {
         let icon_1 = Icon::new(IconName::FileDoc);
         let icon_2 = Icon::new(IconName::FileDoc);
         let icon_3 = Icon::new(IconName::FileDoc);
@@ -658,7 +664,7 @@ impl RenderOnce for IconWithIndicator {
 }
 
 impl ComponentPreview for Icon {
-    fn examples(_cx: &WindowContext) -> Vec<ComponentExampleGroup<Icon>> {
+    fn examples(_cx: &mut WindowContext) -> Vec<ComponentExampleGroup<Icon>> {
         let arrow_icons = vec![
             IconName::ArrowDown,
             IconName::ArrowLeft,