Rework element system, phase 1 (#2881)

This is a deep cut. There's still more work to do until we start
building UI with this. I've approached this as additively as possible,
but I've made a few changes to the rest of the code that I think would
be good to upstream before proceeding too much further.

Most of the interesting pieces are in gpui/playground, which is a
standalone binary that opens a single window and renders a new kind of
element. The layout of these new elements is provided by the taffy
layout engine crate, which conforms to web conventions. The idea is that
playground is relatively cheap to build and work on. As concepts
coalesce in playground, we can drop them into gpui and start
transitioning.

crates/editor/src/editor.rs

@@ -8563,6 +8563,7 @@ fn build_style(
                 font_size,
                 font_properties,
                 underline: Default::default(),
+                soft_wrap: false,
             },
             placeholder_text: None,
             line_height_scalar,