feat: asynchronous timer API

[hargoniX]

This PR implements a basic asynchronous timer API on top of the libuv work.

It purposely puts this into Std.Internal as we might still have to change the API as we continue develop of the async library across releases so I would only like to stabilize it once we are certain this is a fine API.

A few additional notes:

• we currently do not implement a bind operator on AsyncTask on purpose as Task.bind on Task.pure is a non trivial operation and users should be aware of it. Furthermore there is the consideration that as they will have to bind on both IO and AsyncTask we might want to make potential task points explicit in the syntax (did somebody say await?).
• the API generally takes inspiration from https://docs.rs/tokio/latest/tokio/time/index.html, though it has to adapt as Rust's and Lean's asynchronity concepts are sufficiently different.

Stacked on top of #6219.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 00718c3959d93972b43ee30ac008e9d655d9f151 --onto cb600ed9b436e4290b819b0529f8454490bffeb6. (2024-12-04 14:08:17)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 00718c3959d93972b43ee30ac008e9d655d9f151 --onto 019f8e175f9650b829aca8ee501f41c0a5d9076d. (2024-12-05 17:09:27)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 00718c3959d93972b43ee30ac008e9d655d9f151 --onto 6e60d130845855a9c3263c235bcbcc543b9b5c32. (2024-12-06 16:42:58)
• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2025-01-02 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. (2025-01-02 13:44:03)
• 🟡 Mathlib branch lean-pr-testing-6306 build against this PR was cancelled. (2025-01-03 09:46:57) View Log
• ✅ Mathlib branch lean-pr-testing-6306 has successfully built against this PR. (2025-01-03 10:29:39) View Log