Add diagnostic for max_user_watches on Linux

What does this merge request do and why?

This checks that fs.inotify.max_user_watches has a reasonable value on Linux systems.

A low max_user_watches value can cause webpack/Vite/Jest/IDE failures.

How to set up and validate locally

Linux

1. Set fs.inotify.max_user_watches to a low value: sudo sysctl --write fs.inotify.max_user_watches=8192
2. Run gdk doctor
3. Observe warning
4. Set it to a high value: sudo sysctl --write fs.inotify.max_user_watches=524288
5. Run gdk doctor
6. Observe no warning this time

macOS

1. Run gdk doctor
2. Observe no warning

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.