implement file dependency messages

This looks like a massive change because the indent changed by wrapping everything in prepare.

Changed :

use prepare to make sure that each time a plugin instance runs it starts from the same state
deep clone of the UserProps to make sure that there is no weird drift in state during file watching
emit a message that will be picked up by tools like postcss-cli

see : https://github.com/GoogleChromeLabs/postcss-jit-props/issues/24#issuecomment-1378342599

I am unsure how widespread the support for dependency messages through PostCSS is. Some tools might support it, others might not. PostCSS CLI definitely supports it and it is clearly documented in the guidelines for PostCSS

But I personally do not think it is up to plugins to implement file watching through things like chokidar. Better to leave this up to builders, bundlers, meta frameworks, ...

GoogleChromeLabs / postcss-jit-props

implement file dependency messages #32