Fix recent regression with comments right before imports

[alexarchambault]

This fixes a recent regression when a comment precedes an import in a code block, like

@ {
    // hello
    import cats.kernel._
  }

(introduced in #1361, that fixes nice things otherwise)

Before this PR, ones gets:

Welcome to the Ammonite Repl 3.0.0-M0-33-94c5ce87 (Scala 2.13.11 Java 1.8.0_362)
@ import $ivy.`org.typelevel::cats-kernel:2.6.1`
import $ivy.$

@ {
  // hello
  import cats.kernel._
  }
import hello
import cats.kernel._

(note the import hello towards the end)

With this PR, we get the expected output at the end:

Welcome to the Ammonite Repl 3.0.0-M0-35-3a5444f1 (Scala 2.13.11 Java 1.8.0_362)
@ import $ivy.`org.typelevel::cats-kernel:2.6.1`
import $ivy.$

@ {
  // hello
  import cats.kernel._
  }
import cats.kernel._