SI-5408 Prompt after incomplete script paste

Transcript paste mode invites the user to keep typing like
regular paste mode, but really you must enter more transcript.

This matters if the script ends in the middle of incomplete
code that the user wants to complete by hand.

Previously,

```
scala> scala> def f() = {

// Detected repl transcript paste: ctrl-D to finish.

// Replaying 1 commands from transcript.

scala> def f() = {

scala> scala> def f() = {

// Detected repl transcript paste: ctrl-D to finish.

     | }
// Replaying 1 commands from transcript.

scala> def f() = {
}
f: ()Unit
```

Now,

```
scala> scala> def f() = {

// Detected repl transcript. Paste more, or ctrl-D to finish.

// Replaying 1 commands from transcript.

scala> def f() = {
     | 42
     | }
f: ()Int

scala> f()
res0: Int = 42
```

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