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/agent_configuration/manage_profiles_modal.rs

@@ -483,7 +483,7 @@ impl ManageProfilesModal {
 
         let icon = match mode.profile_id.as_str() {
             "write" => IconName::Pencil,
-            "ask" => IconName::MessageBubbles,
+            "ask" => IconName::Chat,
             _ => IconName::UserRoundPen,
         };