feat: experimental active watcher for rollup

[joemckenney]

🔗 Linked issue

#281

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• [] 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Begins work to progress #281.

+ Adds a watch flag to the CLI
+ Adds a watch configuration object to the BuildConfig, proxying Rollup's watch configuration
+ Utilizes rollup.watch to rebuilds your bundle(s) on changes
+ Add basic logging

Open Items/Questions

• Disable and/or ignore stub when watch provided given that these "modes" should be mutually exclusive
• Clean up the build() functions interface (devMode feels sloppy)
• Choose a better strategy for logging what will be rebuilt (using the rollupOptions is limiting)
• Potentially make watch even more rollup-build specific

Note I've tagged this WIP as I still want to iterate on the build interface and logging. Putting this up to get feedback about wether this is directionally correct. I'll update docs and complete any other checklist items once I remove the WIP label.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pi0]

Thanks for the PR dear @joemckenney ❤️

In order to progress I've made a few changes mainly to move the watch logic to each builder kind.

I also agree we should find better way of setting build mode but perhaps we can iterate over that too.