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/theme/mod.rs

@@ -1,4 +1,5 @@
 mod chart;
+mod files;
 mod generative;
 mod media;
 mod palette;
@@ -10,6 +11,7 @@ use gpui::{App, Global, SharedString, WindowAppearance};
 
 use crate::motion;
 pub use chart::ChartSizes;
+pub use files::FileSizes;
 pub use generative::GenerativeSizes;
 pub use media::MediaSizes;
 pub use palette::{Mix, Palette, Syntax};