Graceful Exit for `--watch` Mode

### Feature Request: Dedicated Key for Graceful Exit in `--watch` Mode

Currently, when `gotestsum` is running in `--watch` mode, the only way to exit the process is via a `Ctrl+C` interruption. While `Ctrl+C` is a standard way to interrupt a process, it results in a **non-zero exit code**, which doesn't offer a "graceful" way to exit the program.

I propose adding a key like `q` or `e` that allows users to perform a graceful exit from the `--watch` mode. This would provide a cleaner and more intentional way to stop the process.

If that makes sense for the project, I'm interested in contributing to this feature. I'm a beginner in Go and would appreciate any advice or guidance on implementing this feature.

Start by locating gotestsum's --watch mode and its current Ctrl+C handling. Review how keyboard input and process exit are handled, then identify or add watch-mode coverage that verifies the chosen key exits gracefully with the intended status.

Written by the indexing model from the issue text.