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/github.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-github"><path d="M15 22v-4a4.8 4.8 0 0 0-1-3.5c3 0 6-2 6-5.5.08-1.25-.27-2.48-1-3.5.28-1.15.28-2.35 0-3.5 0 0-1 0-3 1.5-2.64-.5-5.36-.5-8 0C6 2 5 2 5 2c-.3 1.15-.3 2.35 0 3.5A5.403 5.403 0 0 0 4 9c0 3.5 3 5.5 6 5.5-.39.49-.68 1.05-.85 1.65-.17.6-.22 1.23-.15 1.85v4"/><path d="M9 18c-4.51 2-5-2-7-2"/></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="M9.849 14.288v-2.515a3.018 3.018 0 0 0-.629-2.201c1.887 0 3.773-1.258 3.773-3.459.05-.786-.17-1.559-.629-2.2a4.65 4.65 0 0 0 0-2.201s-.629 0-1.886.943a13.533 13.533 0 0 0-5.03 0c-1.259-.943-1.887-.943-1.887-.943a4.35 4.35 0 0 0 0 2.2 3.398 3.398 0 0 0-.63 2.201c0 2.201 1.887 3.459 3.774 3.459a2.965 2.965 0 0 0-.63 2.2v2.516"/><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M6.076 11.773c-2.836 1.258-3.144-1.258-4.402-1.258"/></svg>