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/disconnected.svg

@@ -1,3 +1 @@
-<svg width="12" height="12" viewBox="0 0 12 12" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M2.59892 2.76222C3.14641 2.17067 3.93013 1.80018 4.80011 1.80018C5.91195 1.80018 6.8813 2.40485 7.40066 3.30389C7.68565 3.09577 8.03064 3.00015 8.4 3.00015C9.39372 3.00015 10.1999 3.7895 10.1999 4.80009C10.1999 5.02884 10.1568 5.24633 10.08 5.44882C11.1749 5.67007 11.9999 6.63941 11.9999 7.80001C11.9999 8.48623 11.7112 9.10497 11.233 9.52683L11.8274 9.99557C12.0224 10.1493 12.058 10.4324 11.9043 10.6274C11.7505 10.8224 11.4674 10.858 11.2724 10.7043L0.172556 2.00436C-0.0231882 1.85099 -0.0574997 1.56825 0.0958708 1.37251C0.249241 1.17676 0.531795 1.14245 0.727671 1.29582L2.59868 2.76184L2.59892 2.76222ZM1.82213 4.43635L9.13873 10.1999H2.70017C1.20903 10.1999 0.000248596 8.99059 0.000248596 7.50001C0.000248596 6.32255 0.753414 5.32133 1.80395 4.95196C1.80151 4.90134 1.8002 4.85072 1.8002 4.80009C1.8002 4.67635 1.8077 4.55448 1.82213 4.43635Z" fill="white"/>
-</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="m2 2 12 12M4.269 4.27a4.2 4.2 0 0 0 1.93 7.93h5.1c.267 0 .53-.039.785-.116M13.72 10.7a2.699 2.699 0 0 0-2.42-3.9h-1.074A4.204 4.204 0 0 0 6.8 3.842"/></svg>