Adds a `--defaults`/`-d` flag to `settings show`. This mode will _optionally_ show a
setting's default value. In other words, this does not always print a default value for
every setting.
A default value is not shown when the current value _is_ the default.
Note: some setting types do not print empty or invalid values. For these setting types,
if the default value is empty or invalid, the same elision logic is applied to printing
the default value.
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