The new "Nav" member on a zcl.File is an opaque object that can be
populated by parsers with an object that supports certain interfaces
that are not part of the main API but are useful for integration with
editors and other tooling.
As a first example of this, we replace the hack for getting context in
the diagnostic package with a new ContextString interface, which can
then be optionally implemented by a given parser to return a contextual
string native to the source language.
The indication of specific characters in the source code that are in
error is not yet implemented, but this gets at the main functionality
of printing diagnostics.
Some applications treat an entire directory as a configuration, merging
the configurations from all of the files in a directory and treating them
as one.
MergeFiles supports this idea by wrapping the bodies of the several files.
It's not yet implemented here, but once implemented it will act as an
aggregator of the content of the wrapped bodies, delegating to them for
actual body content and then merging the returned body content in a
well-defined way.
The term "element" is already used for an item from a collection in cty,
so we'll use "block" to talk about the nested blocks in a zcl config to
reduce confusion.
Currently lacking correct support for unicode text segmentation into
grapheme clusters, so it miscounts "Column" in positions. This will be
addressed later.
