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_circle.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="M12.8989 5.77778L11.6434 4.52222C10.6384 3.55068 9.29673 3.00526 7.89893 3C6.57285 3 5.30103 3.52678 4.36343 4.46447C3.78887 5.03901 3.36856 5.73897 3.12921 6.5" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M12.8989 3V5.77778H10.1211" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M3.1012 10.2222L4.3568 11.4778C5.3618 12.4493 6.70342 12.9947 8.10122 13C9.42731 13 10.6991 12.4732 11.6368 11.5355C12.2163 10.956 12.6389 10.2487 12.8772 9.47994" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-<path d="M5.87891 10.2222H3.10111V13" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M12.8989 5.77778L11.6434 4.52222C10.6384 3.55068 9.29673 3.00526 7.89893 3C6.57285 3 5.30103 3.52678 4.36343 4.46447C3.78887 5.03901 3.36856 5.73897 3.12921 6.5" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M12.8989 3V5.77778H10.1211" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M3.1012 10.2222L4.3568 11.4778C5.3618 12.4493 6.70342 12.9947 8.10122 13C9.42731 13 10.6991 12.4732 11.6368 11.5355C12.2163 10.956 12.6389 10.2487 12.8772 9.47994" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M5.87891 10.2222H3.10111V13" stroke="black" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/>
 </svg>