onboarding: Add the AI page (#35351)

This PR starts the work on the AI onboarding page as well as the
configuration modal

Release Notes:

- N/A

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Anthony <anthony@zed.dev>

Cargo.lock

@@ -10923,6 +10923,7 @@ dependencies = [
 name = "onboarding"
 version = "0.1.0"
 dependencies = [
+ "ai_onboarding",
  "anyhow",
  "client",
  "command_palette_hooks",
@@ -10933,7 +10934,10 @@ dependencies = [
  "feature_flags",
  "fs",
  "gpui",
+ "itertools 0.14.0",
  "language",
+ "language_model",
+ "menu",
  "project",
  "schemars",
  "serde",