Expressions can now be evaluated within an "EvalContext", which provides
the variable and function scopes. The JSON implementation of this
currently ignores the context entirely and just returns literal values,
since we've not yet implemented the template language parser that would
be needed for the JSON parser to properly support expressions.
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.
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 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.
