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

[prsabahrami]

This PR introduces a new feature to pixi, pixi watch allowing it to watch files for changes and rerun tasks automatically.
The tasks can be defined as they would have been defined before and hence no changes are needed in the pixi.toml file.
The pixi watch command looks at the globs in the inputs field of a task and runs the task. Then watches the files that match the glob (relative to project root), then kills the previous task and reruns the task command.
For example, if the below is defined in the pixi.toml file:

[tasks]
watch-test = { cmd = "echo test", inputs = ["*.txt"] }

running pixi watch watch-test will print echo to the stdout whenever any text file is updated (or created) in the root directory until the user cancels it by ctrl_c.
I added a FileWatcher struct that uses rayon and notify to handle the file watching.
The expected behaviour for when the set of inputs is empty is for the task to quit as though pixi run was used.

[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.

[ruben-arts]

Could you update the description with the full design idea so I can validate it and see if it works as expected?

[prsabahrami]

@Hofer-Julian updated the description!