Add an option to disable the watcher

[tamasfe]

Currently the vite plugin unconditionally starts a chokidar watcher that causes issues in CI environments:

• inotify limits, CI machines often don't have the need to watch files thus have lower inotify limits that might not be enough
• in some cases the watcher might cause vitest to hang, I haven't confirmed this but I've found that removing the plugin would solve the issue

A simple { watch: false } exposed as an option to the plugin could be enough that would disable file watching, also having !process.env.CI as a default value would also be reasonable.