SI-7715 String inpatternation s"$_" for s"${_}"

In a pattern,

```
scala> implicit class RX(val sc: StringContext) {
     | def rx = sc.parts.mkString("(.+)").r }
defined class RX

scala> "2 by 4" match { case rx"$a by $_" => a }
res0: String = 2

scala> val rx"$_ by $b" = "2 by 4"
b: String = 4
```