Clear and concise description of the problem

As a developer using tsdown in watch mode, I want files specified in the copy option to trigger rebuilds when modified, so that I don't need to manually restart the watch process.

Currently, modifying copied files doesn't trigger a rebuild in watch mode.

Suggested solution

Add the files/globs specified in the copy configuration to the watch mode file watcher. When these files change, trigger a rebuild and re-copy them to the output directory.

Alternative

Add an opt-in watchCopy boolean configuration option, though watching copied files seems like expected default behavior.

Additional context

Minimal reproduction available at https://github.com/schplitt/tsdownCopyWatchRepro

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.