Require `other` for pluralized entries in translation files

[jbourassa]

As of writing, pluralization keys have no requirements – as long as they're present, the linter won't complete.

theme-lint has a rule to enforce other in the default locale: https://github.com/Shopify/theme-lint/blob/239d40432ad20e50f1e7e90bd618e58cdfe09690/linters/i18n.js#L199-L201 -- maybe we can do something similar in MatchingTranslations, but make it possible to opt-out with a parameter on the check?

See: https://github.com/Shopify/theme-check/pull/49#discussion_r536366580