For now, this is the only way to set an attribute, and so attributes can
only be set to literal values.
Later this will be generalized so that this is just a helper wrapper
around a "SetAttribute" method that just uses a given expression, which
then helps by constructing the expression from the value first.
The original prototype of hclwrite tried to track both the tokens and
the AST as two parallel data structures. This quickly exploded in
complexity, leading to lots of messy code to manage keeping those two
structures in sync.
This new approach melds the two structures together, creating first a
physical token tree (made of "node" objects, and hidden from the caller)
and then attaching the AST nodes to that token tree as additional sidecar
data.
The result is much easier to work with, leading to less code in the parser
and considerably less complex data structures in the parser's tests.
This commit is enough to reach feature parity with the previous prototype,
but it remains a prototype. With a more usable foundation, we'll evolve
this into a more complete implementation in subsequent commits.
