Render other tab icons in the start slot (#14683)

This PR reworks the rendering for tab icons to allow us to render all of
the tab icons—not just file icons—in the tab's start slot.

The `Item` trait now has a separate `tab_icon` method that can be used
to indicate what icon should be shown for the tab.

Release Notes:

- N/A

crates/workspace/Cargo.toml

@@ -36,7 +36,6 @@ clock.workspace = true
 collections.workspace = true
 db.workspace = true
 derive_more.workspace = true
-file_icons.workspace = true
 fs.workspace = true
 futures.workspace = true
 gpui.workspace = true