Validate strings as UTF-8 during parsing

src/preserves/private/parsing.nim

@@ -188,6 +188,8 @@ proc parsePreserves*(text: string): Preserve[void] =
     Preserves.String <- Preserves.String:
       var v = Value(kind: pkString, string: newStringOfCap(len($1)))
       unescape(v.string, $1)
+      if validateUtf8(v.string) != -1:
+        raise newException(ValueError, "Preserves text contains an invalid UTF-8 sequence")
       pushStack v
 
     Preserves.charByteString <- Preserves.charByteString: