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/debug_detach.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-unplug"><path d="m19 5 3-3"/><path d="m2 22 3-3"/><path d="M6.3 20.3a2.4 2.4 0 0 0 3.4 0L12 18l-6-6-2.3 2.3a2.4 2.4 0 0 0 0 3.4Z"/><path d="M7.5 13.5 10 11"/><path d="M10.5 16.5 13 14"/><path d="m12 6 6 6 2.3-2.3a2.4 2.4 0 0 0 0-3.4l-2.6-2.6a2.4 2.4 0 0 0-3.4 0Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="none"><g stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" clip-path="url(#a)"><path d="m13 3 2-2M1 15l2-2M4.202 13.53a1.598 1.598 0 0 0 2.266 0L8 11.997 4.003 8 2.47 9.532a1.6 1.6 0 0 0 0 2.265l1.732 1.733ZM5 9l1.5-1.5M7 11l1.5-1.5M8 4.003 11.997 8l1.533-1.532a1.599 1.599 0 0 0 0-2.266L11.798 2.47a1.598 1.598 0 0 0-2.266 0L8 4.003Z"/></g><defs><clipPath id="a"><path fill="#fff" d="M0 0h16v16H0z"/></clipPath></defs></svg>