Updating the peers for Firefox and the firefox-desktop-core-reviewers group

[Dave Townsend]

Hi folks.

While much of the Firefox Desktop front-end is split into the various code modules, we also maintain a set of top-level peers[1] for cases where someone more specific is not available or for changes that cover wide swathes of the code. Molly left the project a short while ago and so we've had an open spot to fill. I'm delighted that Sarah Clements has agreed to fill that spot, congratulations Sarah!

You may also note that we've added a `firefox-desktop-core-reviewers` group in phabricator which has the top-level peers and owners as members. I wouldn't expect most folks to add this to their patch themselves but we will be auto-assigning it to patches that touch certain critical files in Firefox where we want to make sure we take a quick look at changes that may impact startup or security.

These changes actually happened just over a week ago but I'm afraid I failed to actually send the note out, sorry about that!

[1] https://firefox-source-docs.mozilla.org/mots/index.html#desktop-firefox