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/cog.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-cog"><path d="M12 20a8 8 0 1 0 0-16 8 8 0 0 0 0 16Z"/><path d="M12 14a2 2 0 1 0 0-4 2 2 0 0 0 0 4Z"/><path d="M12 2v2"/><path d="M12 22v-2"/><path d="m17 20.66-1-1.73"/><path d="M11 10.27 7 3.34"/><path d="m20.66 17-1.73-1"/><path d="m3.34 7 1.73 1"/><path d="M14 12h8"/><path d="M2 12h2"/><path d="m20.66 7-1.73 1"/><path d="m3.34 17 1.73-1"/><path d="m17 3.34-1 1.73"/><path d="m11 13.73-4 6.93"/></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 12.8a4.8 4.8 0 1 0 0-9.6 4.8 4.8 0 0 0 0 9.6Z"/><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M8 9.2a1.2 1.2 0 1 0 0-2.4 1.2 1.2 0 0 0 0 2.4ZM8 2v1.2M8 14v-1.2M11 13.196l-.6-1.038M7.4 6.962 5 2.804M13.196 11l-1.038-.6M2.804 5l1.038.6M9.2 8H14M2 8h1.2M13.196 5l-1.038.6M2.804 11l1.038-.6M11 2.804l-.6 1.038M7.4 9.038 5 13.196"/></svg>