Improve VueHelper.php to manage parameterized locale strings

VueHelper.php::locales() provides an array of localized strings we pass into JavaScript components. This works well for static strings and (currently) for one instance of a singular or plural string - but not both.

Improve this to manage parameterized strings and also pluralization. For example, for pluralization, return a key to the string that includes its plurality so we can use both versions as needed.

Parameterized strings will require more thought since we may not know the value of the parameter until the component is running. As was done in v4, we may want to create an equivalent string-replacement function for use within components themselves - and look for standard Vuejs ways of doing this.