Revise and clean up some icons (#35582)

This is really just a small beginning, as there are many other icons to
be revised and cleaned up. Our current set is a bit of a mess in terms
of dimension, spacing, stroke width, and terminology. I'm sure there are
more non-used icons I'm not covering here, too. We'll hopefully tackle
it all soon leading up to 1.0.

Closes https://github.com/zed-industries/zed/issues/35576

Release Notes:

- N/A

assets/icons/chat.svg

@@ -0,0 +1,4 @@
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M4.17279 8.26346C4.87566 8.62402 5.68419 8.72168 6.4527 8.53885C7.2212 8.35601 7.89913 7.90471 8.36433 7.26626C8.82953 6.62781 9.0514 5.8442 8.98996 5.05664C8.92852 4.26908 8.58781 3.52936 8.02922 2.97078C7.47064 2.41219 6.73092 2.07148 5.94336 2.01004C5.1558 1.9486 4.37219 2.17047 3.73374 2.63567C3.09529 3.10087 2.64399 3.7788 2.46115 4.5473C2.27832 5.31581 2.37598 6.12435 2.73654 6.82721L2 9L4.17279 8.26346Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+<path d="M7.07168 11C7.16761 11.4537 7.35843 11.8857 7.63567 12.2662C8.10087 12.9047 8.7788 13.356 9.5473 13.5388C10.3158 13.7217 11.1243 13.624 11.8272 13.2634L14 14L13.2635 11.8272C13.624 11.1243 13.7217 10.3158 13.5388 9.54728C13.356 8.77877 12.9047 8.10084 12.2663 7.63564C11.8858 7.3584 11.4537 7.16759 11 7.07166" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>