Add new Button and IconButton components (#3448)

This PR adds new `Button` and `IconButton` components built on top of
our new button abstractions.

Both of these buttons are built from the common `ButtonLike` base, and
implement the `ButtonCommon` (name TBD) trait in order to provide a
common interface.

There are still some visual tweaks that we'll need to make to the new
buttons, but those should be straightforward to make after we land this.

Release Notes:

- N/A

crates/diagnostics2/src/toolbar_controls.rs

@@ -1,5 +1,6 @@
 use crate::ProjectDiagnosticsEditor;
 use gpui::{div, Div, EventEmitter, ParentElement, Render, ViewContext, WeakView};
+use ui::prelude::*;
 use ui::{Icon, IconButton, Tooltip};
 use workspace::{item::ItemHandle, ToolbarItemEvent, ToolbarItemLocation, ToolbarItemView};