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/check_circle.svg

@@ -1,4 +1,4 @@
-<svg width="14" height="14" viewBox="0 0 14 14" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M5 8L6.5 9L9 5.5" stroke="#11181C" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
-<circle cx="7" cy="7" r="4.875" stroke="#11181C" stroke-width="1.25"/>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M5.94873 9.02564L7.48722 10.0513L10.0514 6.46149" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M8 14C11.3137 14 14 11.3137 14 8C14 4.68629 11.3137 2 8 2C4.68629 2 2 4.68629 2 8C2 11.3137 4.68629 14 8 14Z" stroke="black" stroke-width="1.5"/>
 </svg>