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

crates/agent_ui/src/context_picker.rs

@@ -148,7 +148,7 @@ impl ContextPickerMode {
             Self::File => IconName::File,
             Self::Symbol => IconName::Code,
             Self::Fetch => IconName::Globe,
-            Self::Thread => IconName::MessageBubbles,
+            Self::Thread => IconName::Thread,
             Self::Rules => RULES_ICON,
         }
     }