SI-9492 Line trimming paste

Use `-` stripmargin character to indicate trim (i.e.
remove leading indentation). `<<` looks more like
shift left, but is already the standard here doc sequence.

Indentation is often mangled by pasting, so trimming
normalizes lines for error messages. The entire paste
text was already trimmed as a whole.

`-Dscala.repl.here` provides a default end string, which
is unset unless specified.

```
scala> :pa <-
// Entering paste mode (ctrl-D to finish)

def g = 10
     def f! = 27
--

// Exiting paste mode, now interpreting.

<console>:2: error: '=' expected but identifier found.
def f! = 27
     ^
```

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