Support conversion of `\n` to ZPL newline characters `\\&`

[itsmattking]

Create an extension function of String perhaps?

[jbduncan]

I can see at least two options:

1. Automatically translate \n into \\& behind the scenes, as part of the build() function perhaps.
   • Pro: This would let us use Kotlin multiline blocks and trimIndent().
   • Cons:
     • It's less explicit what's happening.
     • If \n means something useful to the printer, we'd need a workaround to support it.
2. Make a function like ZplBuilder.appendLine(), à la https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.text/append-line.html#appendline.
   • Pro: It's explicit what's happening.
   • Con: We wouldn't be able to use Kotlin multiline blocks.