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/chevron_left.svg

@@ -1,3 +1,3 @@
-<svg width="14" height="14" viewBox="0 0 14 14" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M8.35938 3.63281L5.125 6.99344L8.35938 10.3672" stroke="black" stroke-width="1.25" stroke-linecap="round" stroke-linejoin="round"/>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M9.55361 4.15179L5.85718 7.99251L9.55361 11.8482" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>