Files: a tile's name keeps its ends; the ellipses share one line cutter

gpui's line_clamp wraps once and lays the rest of a long name on the
second line, which a centered tile then clipped at both ends. A tile's
name is now one line, cut in the middle so the front and the extension
stay. MiddleEllipsis draws like Ellipsis: in the text style around it,
sized by its text, so a short name centers; the two share one cutter.

examples/gallery/pages/files/mod.rs

@@ -44,6 +44,11 @@ fn entries() -> Vec<DirEntry> {
         DirEntry::file("Walkthrough.mov", 184_000_000, ago(120)),
         DirEntry::file("main.rs", 9_200, ago(1)),
         DirEntry::file("Archive.zip", 48_000_000, ago(700)),
+        DirEntry::file(
+            "Quarterly review with the design team, final.pdf",
+            820_000,
+            ago(6),
+        ),
     ]
 }