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/eye.svg

@@ -1 +1,4 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-eye"><path d="M2.062 12.348a1 1 0 0 1 0-.696 10.75 10.75 0 0 1 19.876 0 1 1 0 0 1 0 .696 10.75 10.75 0 0 1-19.876 0"/><circle cx="12" cy="12" r="3"/></svg>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M2.0375 8.2088C1.9875 8.07409 1.9875 7.92592 2.0375 7.79122C2.5245 6.61039 3.35114 5.60076 4.41264 4.89031C5.47414 4.17986 6.72268 3.8006 7.99999 3.8006C9.2773 3.8006 10.5258 4.17986 11.5873 4.89031C12.6488 5.60076 13.4755 6.61039 13.9625 7.79122C14.0125 7.92592 14.0125 8.07409 13.9625 8.2088C13.4755 9.38962 12.6488 10.3993 11.5873 11.1097C10.5258 11.8202 9.2773 12.1994 7.99999 12.1994C6.72268 12.1994 5.47414 11.8202 4.41264 11.1097C3.35114 10.3993 2.5245 9.38962 2.0375 8.2088Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M8.0001 9.79988C8.99416 9.79988 9.80001 8.99404 9.80001 7.99998C9.80001 7.00592 8.99416 6.20007 8.0001 6.20007C7.00604 6.20007 6.2002 7.00592 6.2002 7.99998C6.2002 8.99404 7.00604 9.79988 8.0001 9.79988Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>