Plural Coverage Audit

Check that every plural message covers the categories its language actually uses. A language with six categories given two has no message at all for the other four, and the usual fallback is the key itself appearing in the interface. Both suffixed keys and ICU plural blocks are read. Also reports a declared category the language never selects, so the text is translated and never shown, and an ICU plural with no other branch, which the format requires. The category table is published.