feat: Add support for auto reload by watching a set of files in tasks

[prsabahrami]

This implements support for adding a set of watched files to each task in Pixi.
The tasks can be defined as below and will run in async watching for changes. The watches can be cancelled by pressing ctrl c.

watch-task = { cmd = "cat watched_file.txt", watched-files = ["watched_file.txt"] }

[bollwyvl]

File-based watching is a magical feature! Some thoughts:

Adding a new schema thing seems a bit unnecessary vs using the existing inputs.

To allow all existing pixi projects to benefit from this without having to rewrite anything, perhaps putting the feature behind a pixi watch (instead of run) would allow for reusing the task.taskname.inputs, and avoid surprises on e.g. CI where a watching task that never finishes would be a pretty big problem.

This further introduces the challenge of depends-on in the case of watching.

One option might be to gather the whole list of watched globs -> tasks, and rerun the correct one, in order, when their appropriate input changes. This behavior could be disabled via e.g. pixi watch --skip-deps taskname.