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_right_left.svg

@@ -1,6 +1,6 @@
 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M11 2L13 4.5L11 7" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M12.5 4.5H2.5" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M5 14L3 11.5L5 9" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M3 11.5H13" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M11 2L13 4.5L11 7" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M12.5 4.5H2.5" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M5 14L3 11.5L5 9" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M3 11.5H13" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>