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.
Its implementation calls into each of the child bodies in turn and merges
the result to produce a single BodyContent. This is intended to support
the case of a directory being the unit of configuration rather than a
file, with the calling application discovering and parsing each of the
files in its workspace and then merging them together for processing as
a single configuration.
