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/book_copy.svg

@@ -1 +1 @@
-<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-book-copy"><path d="M2 16V4a2 2 0 0 1 2-2h11"/><path d="M5 14H4a2 2 0 1 0 0 4h1"/><path d="M22 18H11a2 2 0 1 0 0 4h11V6H11a2 2 0 0 0-2 2v12"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="none"><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M2.5 10.5V3.643c0-.303.113-.594.315-.808.202-.215.476-.335.762-.335H9.5"/><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M4.5 9.5h-.667c-.353 0-.692.105-.942.293-.25.187-.391.442-.391.707 0 .265.14.52.39.707.25.188.59.293.943.293H4.5M13.5 11.25H7.577c-.286 0-.56.118-.762.33a1.151 1.151 0 0 0-.315.795m0 0c0 .298.113.585.315.796.202.21.476.329.762.329H13.5v-9H7.577c-.286 0-.56.119-.762.33a1.151 1.151 0 0 0-.315.795v6.75Z"/></svg>