Watch mode: kill current build and restart

Re-creating https://github.com/rollup/rollup/issues/5795 which was kinda closed in https://github.com/rollup/rollup/pull/5799 but only considered plugins.

Feature Use Case

Our build process takes quite some time and if multiple files are changed, the first build starts, waits until it finished and starts again because other changes happened in the meantime.

Feature Proposal

Add a watch configuration option to enable the feature to kill the current build and restart it when a file is modified during a build.

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I would've used nodemon, but I will lose rollup feature that tracks all the included files automatically. With nodemon I might forget to include some paths, or include some extra paths, etc.