This new translation context is for some special cases when
translation cannot be avoided, for example in an interface where some
props are built-in (translatable) and others are
user-defined (non-translatable), but we don't know which ones in
advance.
It allows specifying explicitly that translation should not occur
for user data when building the UI.
It is a followup to !145963, in which the context was introduced as a
string literal instead of a defined context.
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