Add UserLocalizedString and SystemLocalizedString for i18n

Currently i18n'd strings that are defined at compile time only return
english text. This occurs with both system and user locale
strings. For system locale, the locale can be changed via setting, so
we can't rely on the JVM locale on startup to be correct as it may
have changed. For the user locale, it could be different for every
request.

This commit adds two new `tru` and `trs` macros that have the same
name as the `puppetlabs.i18n.core` macros (and thus will be extracted
similarly) but will return a defrecord with a `toString` method. This
delays the conversion of the english text to the translated text until
`str` is invoked. All of the code hasn't been flipped to use the new
i18n macros yet, but this gets it in place and tested for the rest of
the application to be migrated to.

Fixes #8245