Add watch mode support for target files

[uraitakahito]

Add watch mode support for target files

What does this PR do?

The following changes have been made:

• Added a feature to watch for changes in files specified in targets' src during watch mode.
• Only changes in input and targets' src are detected. In other words, targets' dest is not monitored.
• This feature is OFF by default to maintain compatibility with the current behavior.
• Adds relevant tests to ensure the new feature works as expected.

Motivation

Because I want rollup to automatically rerun when assets other than JavaScript are changed during web application development.

Describe how you validated your changes

I have confirmed that all existing tests pass and added new tests. Additionally, I created a repository for testing, which can be found here.

Possible Drawbacks / Trade-offs

The current implementation re-copies all targets instead of just the modified file, which is inefficient. Users should be aware of this limitation. I hope to address this in a separate pull request if possible.

Additional Notes

• addWatchFile is prohibited from being called in the buildEnd event. Therefore, the registration of watch targets is fixed to the buildStart event.
• A temporary package with the new feature is available here.