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_up.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="M3.63281 8.36719L6.99344 5.13281L10.3672 8.36719" 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="M4.15186 9.56252L7.99258 5.86609L11.8483 9.56252" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>