fix(watch-mode): allow run plugin in watch mode

[IvanRodriCalleja]

Fixes: https://github.com/StarkShang/vite-plugin-chrome-extension/issues/6

The problem with the watch mode was that the input for content script was not regenerated and the new manifest was wrong generated because the content script was not found in the bundle as a file.

@StarkShang Would you be open to review and release a new version in case you are agree with the fix or do you prefer that i fork the repo and maintain my own plugin?