Bug 1556393 - Pre 1 (m-c): Update .hg-format-source to use Prettier when running format-source, r=andi, CLOSED TREE

Differential Revision: https://phabricator.services.mozilla.com/D36719

--HG--
extra : source : 63ae0f90503d187bd2a1c4bf49874f538de3768c
extra : amend_source : 5d828afe9c0ef3cff55c0038a1dbc1621b52b519
extra : intermediate-source : 18d01e76936799b473fdce30bb702dcc92753c9b