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/arrow_down.svg

@@ -1,3 +1,3 @@
 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M8 13L12.5 8.5M8 13L3.5 8.5M8 13V3" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M8 13L12.5 8.5M8 13L3.5 8.5M8 13V3" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>