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

Cargo.lock

@@ -3601,6 +3601,7 @@ dependencies = [
  "db",
  "emojis",
  "env_logger",
+ "file_icons",
  "futures 0.3.28",
  "fuzzy",
  "git",
@@ -13336,7 +13337,6 @@ dependencies = [
  "derive_more",
  "dev_server_projects",
  "env_logger",
- "file_icons",
  "fs",
  "futures 0.3.28",
  "gpui",