Redesign and clean up all icons across Zed (#35856)

- [x] Clean up unused and old icons
- [x] Swap SVG for all in-use icons with the redesigned version
- [x] Document guidelines

Release Notes:

- N/A

assets/icons/file.svg

@@ -1,4 +1 @@
-<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M10.0001 1.33334H4.00008C3.64646 1.33334 3.30732 1.47382 3.05727 1.72387C2.80722 1.97392 2.66675 2.31305 2.66675 2.66668V13.3333C2.66675 13.687 2.80722 14.0261 3.05727 14.2762C3.30732 14.5262 3.64646 14.6667 4.00008 14.6667H12.0001C12.3537 14.6667 12.6928 14.5262 12.9429 14.2762C13.1929 14.0261 13.3334 13.687 13.3334 13.3333V4.66668L10.0001 1.33334Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M9.33325 1.33334V4.00001C9.33325 4.35363 9.47373 4.69277 9.72378 4.94282C9.97383 5.19287 10.313 5.33334 10.6666 5.33334H13.3333" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="none"><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M9.875 2H4.25c-.332 0-.65.126-.884.351-.234.226-.366.53-.366.849v9.6c0 .318.132.623.366.849.235.225.552.351.884.351h7.5c.332 0 .65-.127.884-.351.234-.225.366-.53.366-.85V5L9.875 2Z"/><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M9 2v2.667A1.333 1.333 0 0 0 10.333 6H13"/></svg>