Remove hard dependency on IntlMessageFormat

This removes the hard dependency on `IntlMessageFormat` in favor of pushing people towards first creating `IntlMessageFormat` instances which they then provide to Handlebars via `data.intl`.

People should not be putting literal string messages into their templates because these messages are locale-specific, instead they `IntlMessageFormat` instances should be created once, ahead of time, and then used by Handlebars during template rendering. The whole idea of this i18n suite of packages is to extract the locale-specific strings from the application's templates, code, etc.

App developers will have a collection of string-based messaged per-locale in their code repositories. When their app initializes, those string messages should be converted into `IntlMessageFormat` instances, and then used throughout the app, including used via this package.