Rather than setting an "Unwrap" field on the existing TemplateExpr, we'll
instead use a separate, simpler AST node.
There is very little in common between these two cases, so overloading the
TemplateExpr node doesn't buy much. A separate node is needed here,
rather than just returning the wrapped node directly, to give us somewhere
to capture the full source range extent of the wrapping template, whereas
the inner expression only captures the range of itself. This is important
both for good diagnostics and for transforming zclsyntax AST into
zclwrite AST.
Previously we were detecting the exactly-one-part case at parse time and
skipping the TemplateExpr node in the AST, but this was problematic
because we then misaligned the source range for the expression to the
_content_ of the quotes, rather than including the quotes themselves.
As well as producing confusing diagnostics, this also caused problems for
zclwrite since it relies on source ranges to map our AST back onto the
source tokens it came from.
