Clear and concise description of the problem

As a developer using Vite I wish to --watch for changes to project files in /public (and/or any other root subdirectory) and transpose them to /dist upon change. This would speed up dev time and create a more efficient workflow when building for multiple platforms (ie: cms/ecommerce theme development) from a single codebase.

Suggested solution

During --watch, add an option in vite.config.js to specify directories to monitor for changes and possibly specify an argument for what to do with those files (transpile, transpose, etc).

Alternative

Coming from webpack, this would be a simple thing to add to webpack.config.js -> https://webpack.js.org/configuration/watch/

Additional context

Possibly somewhat related use-case... #3233 (comment)

Validations

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