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/files/mod.rs

@@ -0,0 +1,10 @@
+mod grid;
+mod icon;
+mod item;
+
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
+
+pub use grid::FileGrid;
+pub use icon::FileIcon;
+pub use item::FileItem;