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/blocks.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-blocks-icon lucide-blocks"><rect width="7" height="7" x="14" y="3" rx="1"/><path d="M10 21V8a1 1 0 0 0-1-1H4a1 1 0 0 0-1 1v12a1 1 0 0 0 1 1h12a1 1 0 0 0 1-1v-5a1 1 0 0 0-1-1H3"/></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="M12.429 2H9.57A.571.571 0 0 0 9 2.571V5.43c0 .315.256.571.571.571h2.858A.571.571 0 0 0 13 5.429V2.57A.571.571 0 0 0 12.429 2ZM6.5 13V4.643A.643.643 0 0 0 5.857 4H2.643A.643.643 0 0 0 2 4.643v7.714a.643.643 0 0 0 .643.643h7.714a.643.643 0 0 0 .643-.643V9.143a.643.643 0 0 0-.643-.643H2"/></svg>