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/close.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="M9.82843 4.17157L4.17157 9.82842M9.82843 9.82842L4.17157 4.17157" stroke="black" stroke-width="1.25" stroke-linecap="round"/>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M4.70581 4.5L11.294 11.5M11.294 4.5L4.70581 11.5" stroke="black" stroke-width="1.5" stroke-linecap="round"/>
 </svg>