JLine 3: fix //print

[SethTisue]

it sort of works but it ends up mangled:

scala> 2 + 2 //prin(2).+(2) // : Int(4)

[SethTisue]

ReplCompletion.scala:

    // secret handshakes
    val slashPrint  = """.*// *print *""".r
    val slashPrintRaw  = """.*// *printRaw *""".r
    val slashTypeAt = """.*// *typeAt *(\d+) *(\d+) *""".r

might as well fix all three at once

but shhhh, they're SECRET

[SethTisue]

I've added a commit to scala/scala#8848 that handles this at least reasonably okay