A sequence like "foo" is represented in the AST as a TemplateExpr with a
single string literal inside rather than as a string literal node
directly, so we need to recognize that situation during parsing and treat
it as a special case so we can get the intended behavior of representing
that index as a traversal step rather than as a dynamic index operation.
Most of the time this distinction doesn't matter, but it's important for
static analysis use-cases. In particular, hcl.AbsTraversalForExpr will now
accept an expression like foo["bar"] where before it would've rejected it.
This also includes a better error message for when an expression cannot be
recognized as a single traversal. There isn't really any context here to
return a direct reference to the construct that was problematic, which is
what we'd ideally do, but at least this new message includes a summary
of what is allowed and some examples of things that are not allowed as an
aid to understanding what "static variable reference" means.
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