Syntax start/end patterns can be simplified in escape != 'none' case

[bpstahlman]

Low priority, but while refactoring the syntax definitions, I discovered that the escape concealment matches (which are defined after and hence take priority over the highlight regions) obviate the need for special escaping in b:txtfmt_re_any_tok and friends... Note: This change would be made mostly for aesthetic reasons: the performance gains would probably not be noticed, especially since it affects only non-default settings of 'escape', which I suspect very few (if any) users are using...