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/debug_step_out.svg

@@ -1,5 +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-up-from-dot">
-  <path d="m3 10 9-8 9 8"/>
-  <path d="M12 17V2"/>
-  <circle cx="12" cy="21" r="1"/>
-</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="M3.333 6 8 1.333 12.667 6M8 10.667V1.333"/><path fill="#000" stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.2" d="M8 13.333a.667.667 0 1 1 0 1.334.667.667 0 0 1 0-1.334Z"/></svg>