ui: Reduce icons stroke width (#36361)

After redesigning all Zed icons
(https://github.com/zed-industries/zed/pull/35856), it felt like using
1.5 for stroke width didn't really flow well with the default typeface
default font weight. Reducing it to 1.2 makes the UI much sharper, less
burry, and more cohesive overall.

Release Notes:

- N/A

assets/icons/envelope.svg

@@ -1,4 +1,4 @@
 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-    <path d="M13 4C13.5523 4 14 4.44772 14 5V11C14 11.5523 13.5523 12 13 12H3C2.44772 12 2 11.5523 2 11V5C2 4.44772 2.44772 4 3 4H13Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-    <path d="M13.5 5L7.9999 8.5L2.5 5" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M13 4C13.5523 4 14 4.44772 14 5V11C14 11.5523 13.5523 12 13 12H3C2.44772 12 2 11.5523 2 11V5C2 4.44772 2.44772 4 3 4H13Z" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M13.5 5L7.9999 8.5L2.5 5" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>