This commit has no behavior changes.
In particular, this does not fix any of the wrong uses of the previous
default parameter (which used to be 'false', meaning "only replace the
first occurence in the string"). It simply replaces the default uses by
String::replace(..., ReplaceMode::FirstOnly), leaving them incorrect.
This works largely the same as the PropertyID and ValueID generators,
but using LibMain, Core::Stream, and TRY().
Rather than have a MediaFeatureID::Invalid, I decided to return an
Optional. We'll see if that turns out better or not. :^)
