use string interpolation. fix warnings

[xuwei-k]

[warn] scalaxb/cli/src/main/scala/scalaxb/compiler/xsd/ContextProcessor.scala:228:47: method + in class Char is deprecated (since 2.13.0): Adding a number and a String is deprecated. Use the string interpolation `s"$num$str"`
[warn]       .map {str => if (numbers(str.head)) 'n' + str else str}  // Package name can't start with a number
[warn]                                               ^

[warn] scalaxb/cli/src/main/scala/scalaxb/compiler/xsd/Lookup.scala:164:54: method any2stringadd in object Predef is deprecated (since 2.13.0): Implicit injection of + is deprecated. Convert to String to call +
[warn]     if (!context.typeNames.contains(decl)) sys.error(pkg + ": Type name not found: " + decl.toString)
[warn]                                                      ^
[warn] scalaxb/cli/src/main/scala/scalaxb/compiler/xsd/Lookup.scala:186:17: method any2stringadd in object Predef is deprecated (since 2.13.0): Implicit injection of + is deprecated. Convert to String to call +
[warn]       sys.error(pkg + ": Type name not found: " + enum.toString)
[warn]                 ^

[eed3si9n]

Thanks!