This allows code that only deals with BodyContent (post-decoding) to
still be able to report on missing items within the associated body while
providing a suitable source location.
When producing diagnostics about missing attributes or bodies it's
necessary to have a range representing the place where the missing thing
might be inserted.
There's not always a single reasonable value for this, so some liberty
can be taken about what exactly is returned as long as it's somewhere
the user can relate back to the construct producing the error.
The Content and PartialContent methods deal with the case where the caller
knows what structure is expected within the body, but sometimes the
structure of a body is just a free-form set of attributes that the caller
needs to enumerate.
The idea here is that the block in question must contain only attributes,
and no child blocks. For JSON this just entails interpreting every
property as an attribute. For native syntax later this will mean
producing an error diagnostic if any blocks appear within the body.
This is a wrapper around Body.PartialContent that generates additional
error diagnostics if any object properties are left over after decoding,
helping the config author to catch typos that would otherwise have caused
a property to be silently ignored.
