i18n: adds Occitan locale

[ensag-dev]

I hope it's okay to contribute like this

[catamphetamine]

In order to even consider adding oc language to javascript-time-ago, it should first be added to relative-time-format.
oc is not present in the list of the locales there.
https://github.com/catamphetamine/relative-time-format/tree/master/locale

For example, someone has added Esperanto language there:
https://github.com/catamphetamine/relative-time-format/tree/master/locale-other/eo