Port to PostCSS 8 API.

[ludofischer]

Hi, by trying to port the plugin to use the more efficient PostCSS 8 API, I’ve noticed there’s some confusing logic inside the plugin that prevents this. In the end, as it is, this pull request does not add much except maybe future proofing, but I thought I’d share it to save you some time if you ever try porting yourself.
If I replace the walkDecls() call with Declaration(), the plugin goes into an infinite loop when I run the tests. The PostCSS migration guide says

plugins will re-visit all changed or added nodes. You should check if your transformations were already applied and if that is the case—ignore those nodes. Only Root and RootExit listeners will be called exactly once.

I believe bug4() sees the same node again after bug6() processes it and adds the 0% flex basis value that bug6() then removes again and so on. Because I am not sure I understand the intention adding then removing the 0%, I went back to walking the tree with the Root listener.

[luisrudge]

Thanks for the PR! Released postcss-flexbugs-fixes@5.0.0