Update moz.l10n, add hacky verbose resource diff

[eemeli]

Added a developer utility --verbose option to .github/scripts/diff.py which isn't used in CI, but when used prints a very verbose diff of resources that differ. To do that, it's abusing pytest internals (not a dependency, needs to be installed separately).

The code is ugly enough that it didn't seem like a good idea to put it in moz.l10n.

Updating moz.l10n also revealed that prior to mozilla/moz-l10n#21, standalone .properties comments had been attached to the subsequent entry. Here, they're separated out again like they should be.

@bcolsson Merging this will also bring in mozilla/moz-l10n#26, which drops the NUMBER() from Fluent selectors. Therefore, it would probably be a good idea to pause sync while merging this, and to run l10n-fix here to apply the change to all files.

In firefox-l10n, translated messages can be migrated with something like this:

git grep -l 'NUMBER.*->' | xargs l10n-fix

Currently, that's resulting in 109 changed files when I run it.