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_down_right.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-arrow-down-right-icon lucide-arrow-down-right"><path d="m7 7 10 10"/><path d="M17 7v10H7"/></svg>
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M4.25 4.25L11.125 11.125" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M11.75 4.25006V11.7501H4.25" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>