LibUnicode: Parse the CLDR's defaultContent.json locale list

This file contains the list of locales which default to their parent
locale's values. In the core CLDR dataset, these locales have their own
files, but they are empty (except for identity data). For example:

https://github.com/unicode-org/cldr/blob/main/common/main/en_US.xml

In the JSON export, these files are excluded, so we currently are not
recognizing these locales just by iterating the locale files.

This is a prerequisite for upgrading to CLDR version 40. One of these
default-content locales is the popular "en-US" locale, which defaults to
"en" values. We were previously inferring the existence of this locale
from the "en-US-POSIX" locale (many implementations, including ours,
strip variants such as POSIX). However, v40 removes the "en-US-POSIX"
locale entirely, meaning that without this change, we wouldn't know that
"en-US" exists (we would default to "en").

For more detail on this and other v40 changes, see:
https://cldr.unicode.org/index/downloads/cldr-40#h.nssoo2lq3cba

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