Update first run onboarding

We need to evolve our onboarding flows now that we support sign in with GHCP, third party subscriptions, and BYOK. This is the first interaction users see after install VS Code from the website so the experience and funnel should be cohesive and users can choose how they wish to interact with VS Code moving forward.

Proposed designs from @eli-w-king:

Editor Window

https://github.com/user-attachments/assets/f77c9ea5-1a19-40a3-92b9-b6819637583a

Notes:
- Main changes are to the first page - highlighting the different sign in options.
- I think we can remove the "You're all set" slide at the end. We have seen variants of this be unsuccessful as users prefer contextual onboarding and we want to limit first run onboarding to the "essentials"
- This prototype also features an "import from" step from other editors. This is not currently implemented in our onboarding experience but we want to reduce the switching costs for users that work across products. This would only show if we detected another editor on their machine. This concept was explored in https://github.com/microsoft/vscode/pull/325311
- The icon next to "Sign in with Google" represents GHE. We should switch to "GHE" so this is more clear.

Agents Window

https://github.com/user-attachments/assets/9b323399-21b9-4b77-a77f-ee14df2664aa

Attaching the current state for reference:

https://github.com/user-attachments/assets/c0a8874b-0c09-40e4-b3df-df895682c916