hclsyntax: Tailored error for "curly quotes"

It seems to be somewhat common for someone to share HCL code via a forum
or a document and have the well-meaning word processor or CMS replace the
straight quotes with curly quotes, which then lead to confusing errors
when someone copies the result and tries to use it as valid HCL
configuration.

Here we add a special hint for that, giving a tailored error message
instead of the generic "This character is not used within the language"
error message.

HCL has always had some of these special hints implemented here, and they
were originally implemented with special token types to allow the parser
handle them. However, we later refactored to do the check all at once
inside the Lex* family of functions, prior to parsing, so it's now
relatively straightforward to handle it as a special case of TokenInvalid
rather than an entirely new token type. Perhaps later we'll rework the
existing ones to also just use TokenInvalid, but that's a decision for
another day.

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