Support default locale to allow fallback if message ID not existent in target locale

[threequartersjohn]

For example, a webpage has two locales, en and fr and was first developed with en with future translations in mind, the website will probably have partial fr at various points in development, e.g. for testing implementation of next-intl.

Supporting a fallback feature would make it so message IDs that aren't present in the fr locale would fallback to their respective messages in en, after this locale had been set as the fallback default.