Clarify diagnostics for quote runs at string boundaries

Objo escapes a double quote inside an ordinary string with \"; doubling quotes ("") is only interpreted inside verbatim %"..." strings. When code uses Xojo-style doubled quotes at a string boundary — for example "String """ or """ is not valid." — the compiler reports either Multiline string literal must start with a line ending after the opening """ (the quote run is taken as a multiline-string opener) or a generic parse error such as Expected ')' after arguments., neither of which explains what to write instead. Improve these diagnostics so the message at the failure site also explains how to embed a quote (\"), that "" is an empty string rather than an escape, and that doubled quotes only apply to verbatim strings.