Extending watch system

[d-corler]

In Vite / Rollup, it's straightforward to programmatically add files to the watch list using the addWatchFile method available in the plugin context.

I see tsdown uses its own watch system. When I populate the watch config property it's mostly working, but I have tons of logs that list watched files, and usage of this property overwrite CLI --watch argument so I need to create my own on/off toggle.

Are there any plans to expose a more flexible watch configuration API ?

[sxzz]

Let's wait #329