Add icons crate (#27447)

This PR adds a new `icons` crate and moves the `IconName` into it.

We have a number of crates that are taking a dependency on `ui` just so
they can talk about icons, which is not ideal.

Release Notes:

- N/A

Cargo.toml

@@ -70,6 +70,7 @@ members = [
     "crates/html_to_markdown",
     "crates/http_client",
     "crates/http_client_tls",
+    "crates/icons",
     "crates/image_viewer",
     "crates/indexed_docs",
     "crates/inline_completion",
@@ -274,6 +275,7 @@ gpui_tokio = { path = "crates/gpui_tokio" }
 html_to_markdown = { path = "crates/html_to_markdown" }
 http_client = { path = "crates/http_client" }
 http_client_tls = { path = "crates/http_client_tls" }
+icons = { path = "crates/icons" }
 image_viewer = { path = "crates/image_viewer" }
 indexed_docs = { path = "crates/indexed_docs" }
 inline_completion = { path = "crates/inline_completion" }