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/eraser.svg

@@ -1,4 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-eraser">
-    <path d="m7 21-4.3-4.3c-1-1-1-2.5 0-3.4l9.6-9.6c1-1 2.5-1 3.4 0l5.6 5.6c1 1 1 2.5 0 3.4L13 21"/>
-    <path d="M22 21H7"/><path d="m5 11 9 9"/>
-</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="m5.015 12.983-2.567-2.382c-.597-.554-.597-1.385 0-1.884L8.179 3.4c.597-.554 1.493-.554 2.03 0L13.552 6.5c.597.554.597 1.385 0 1.884l-4.955 4.598M14 12.983H5M4.5 7.483l5 5"/></svg>