Extract UI elements from storybook into new ui crate (#3008)

This PR extracts the various UI elements from the `storybook` crate into
a new `ui` library crate.

Release Notes:

- N/A

crates/ui/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "ui"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+anyhow.workspace = true
+gpui2 = { path = "../gpui2" }
+serde.workspace = true
+settings = { path = "../settings" }
+theme = { path = "../theme" }