Asyncronous prompt updates

[philocalyst]

Summary

So this fixes my own issue #1045. It's meant to be composed with an upcoming Nushell PR, which I will link here when I have it up. My biggest motivation is that I'm working on changes to starship that should bring these different kinds of prompts to users, and nushell is my daily driver. Wanted it here too, of course :0

Before

Before this, there existed no way to repaint the prompt after the first pass.

After

After this, the mechanism exists for repainting, opening use-cases for things like:

• An accurate time prompt
• Asynchronus git status (non-blocking)
• Gradient madness??

Additional notes

• I noticed that other features, like external_printer are gated, not sure if that should be the case here too.
• We don't have any real integration tests against a PTY which I was surprised to see, which makes true testing here hard, but I did verify myself.

[Copilot]

Pull request overview

Adds an asynchronous repaint mechanism to Reedline so the prompt can be re-evaluated and redrawn in-place while read_line() is active, enabling non-blocking prompt updates (e.g., async git status).

Changes:

• Introduces a new public RepaintSignal handle for requesting a prompt repaint from another thread.
• Updates the input loop to periodically poll for repaint requests (and repaint when requested).
• Adds unit tests validating repaint request consumption, handle sharing behavior, and polling activation.

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[philocalyst]

Resolved comments