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/cloud_download.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-cloud-download-icon lucide-cloud-download"><path d="M12 13v8l-4-4"/><path d="m12 21 4-4"/><path d="M4.393 15.269A7 7 0 1 1 15.71 8h1.79a4.5 4.5 0 0 1 2.436 8.284"/></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="M8.001 9v4l-2-2M8.001 13l2-2"/><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M3.436 10.389a4.215 4.215 0 0 1-1.424-3.484 4.227 4.227 0 0 1 1.92-3.236 4.19 4.19 0 0 1 5.335.665 4.22 4.22 0 0 1 .96 1.677H11.3c.584 0 1.151.19 1.618.54a2.71 2.71 0 0 1 .913 3.116A2.709 2.709 0 0 1 12.762 11"/></svg>