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.lock

@@ -6490,6 +6490,14 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "icons"
+version = "0.1.0"
+dependencies = [
+ "serde",
+ "strum",
+]
+
 [[package]]
 name = "icu_collections"
 version = "1.5.0"
@@ -14928,6 +14936,7 @@ dependencies = [
  "chrono",
  "component",
  "gpui",
+ "icons",
  "itertools 0.14.0",
  "linkme",
  "menu",