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/arrow_down10.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-arrow-down10-icon lucide-arrow-down-1-0"><path d="m3 16 4 4 4-4"/><path d="M7 20V4"/><path d="M17 10V4h-2"/><path d="M15 10h4"/><rect x="15" y="14" width="4" height="6" ry="2"/></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="m2.5 10.667 2.667 2.666 2.666-2.666M5.167 13.333V2.667M11.833 6.667v-4H10.5M10.5 6.667h2.667M13.167 10.667a1.333 1.333 0 0 0-2.667 0V12a1.333 1.333 0 0 0 2.667 0v-1.333Z"/></svg>