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/database_zap.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-database-zap"><ellipse cx="12" cy="5" rx="9" ry="3"/><path d="M3 5V19A9 3 0 0 0 15 21.84"/><path d="M21 5V8"/><path d="M21 12L18 17H22L19 22"/><path d="M3 12A9 3 0 0 0 14.59 14.87"/></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="M8.017 5.625c2.974 0 5.385-.804 5.385-1.795 0-.991-2.41-1.795-5.385-1.795-2.973 0-5.384.804-5.384 1.795 0 .991 2.41 1.795 5.384 1.795Z"/><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M2.633 3.83v8.376c-.003.288.201.571.596.827.394.256.967.477 1.671.643a13.12 13.12 0 0 0 2.373.314c.854.04 1.725.01 2.54-.085M13.402 3.83v1.795M13.402 8.018l-1.795 2.991H14l-1.795 2.992"/><path stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M2.633 8.018c0 .28.198.556.575.805.378.25.925.467 1.599.634.673.167 1.454.279 2.28.327.827.048 1.676.032 2.48-.049"/></svg>