Provide 'upgrade' function to polyfill data from previous versions

[bartveneman]

This package has had some breaking changes in the past and it's hard to compare results from previous analysis with the current ones, for example using css-analyzer-diff.

This library should export a function to upgrade an old report to the latest data structure so it can be used across places like projectwallace.com (for example on a diff page), or in the CSS Diff GitHub Action.