-ltr and -rtl-prefixes on rules don't work with less@3.12.2

[eulbot]

I tried to debug a little, looks like the plugin's visitRule function is never invoked.
I've added a basic test repo: https://github.com/eulbot/less-plugin-rtl-test

[eulbot]

Found it, Rule has been renamed to Declaration, see here.

I've opened a PR.