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/arrow_circle.svg

@@ -1,6 +1,6 @@
 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M3 8C3 6.67392 3.52678 5.40215 4.46446 4.46447C5.40214 3.52679 6.67391 3.00001 7.99999 3.00001C9.39779 3.00527 10.7394 3.55069 11.7444 4.52223L13 5.77778" stroke="black" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M13 3.00001V5.77778H10.2222" stroke="black" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M13 8C13 9.32608 12.4732 10.5978 11.5355 11.5355C10.5978 12.4732 9.32607 13 7.99999 13C6.60219 12.9947 5.26054 12.4493 4.25555 11.4778L3 10.2222" stroke="black" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M5.77777 10.2222H3V13" stroke="black" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M3 5.77778L4.25556 4.52222C5.26054 3.55068 6.6022 3.00526 8 3C9.32608 3 10.5979 3.52678 11.5355 4.46447C12.2339 5.16285 12.7044 6.04656 12.899 7" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M3 3V5.77778H5.77778" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M13.0001 10.2222L11.7445 11.4778C10.7395 12.4493 9.39788 12.9947 8.00008 13C6.67399 13 5.40222 12.4732 4.46454 11.5355C3.76616 10.8372 3.29571 9.95344 3.10107 9" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M10.2224 10.2222H13.0002V13" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>