Files · Items: one icon map, file icon, file row and file grid

The crate had two maps from a file's name to its icon, in chat and in the
file tree, and they disagreed. One map now lives in primitives, over the
Lucide file-* icons, and chat, agent, the file tree and the directory
listing use it. FileIcon draws it for a file or a folder. FileItem is a
file's row, a ListItem with its size and when it changed. FileList is the
directory listing. FileGrid lays tiles in the order given, with a cursor
seeded from the owner's pick: arrows by tile and by row, Home, End, Enter
and a double press to open. The count of what fits a width has one home,
layout's columns_for, and fit builds on it.

src/agent/changes.rs

@@ -8,11 +8,10 @@ use gpui::{
 
 use crate::{
     buttons::{Button, ButtonVariant},
-    chat::file_icon,
     collab::Decision,
     forms::OnFlag,
     git::{DiffStat, DiffViewer, stat},
-    primitives::{Disclosure, FocusRing, Icon, IconName},
+    primitives::{Disclosure, FocusRing, Icon, IconName, file_icon},
     theme::{ActiveTheme, ControlSize, IconSize, Radius, TextSize},
     typography::Ellipsis,
 };