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/backspace.svg

@@ -1,3 +1,5 @@
-<svg width="15" height="11" viewBox="0 0 15 11" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M5.24432 11L0.183239 5.90909L5.24432 0.818182H14.75V11H5.24432ZM5.68679 9.90625H13.6761V1.91193H5.68679L1.70952 5.90909L5.68679 9.90625ZM11.7223 8.15625L10.9964 8.89205L5.75639 3.66193L6.48224 2.92614L11.7223 8.15625ZM6.48224 8.89205L5.75639 8.15625L10.9964 2.92614L11.7223 3.66193L6.48224 8.89205Z" fill="black"/>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M6.79998 4C6.50183 4.00002 6.21436 4.10574 5.99358 4.29657L2.19677 7.57657C2.1348 7.63013 2.08528 7.69545 2.05139 7.76832C2.01751 7.8412 2 7.92001 2 7.99971C2 8.07941 2.01751 8.15823 2.05139 8.23111C2.08528 8.30398 2.1348 8.36929 2.19677 8.42286L5.99358 11.7034C6.21436 11.8943 6.50183 12 6.79998 12H12.8C13.1183 12 13.4235 11.8796 13.6485 11.6653C13.8736 11.4509 14 11.1602 14 10.8571V5.14286C14 4.83975 13.8736 4.54906 13.6485 4.33474C13.4235 4.12041 13.1183 4 12.8 4H6.79998Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M8.5 6.5L11.5 9.5" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M11.5 6.5L8.5 9.5" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>