SI-8173 add support for patterns like init :+ last to quasiquotes

Adds support for patterns like:
```scala
  val q"{ ..$init; $last }" = q"{ a; b; c }"
  // init == List(q"a", q"b")
  // last == q"c"
```
Which under the hood get compiled as `:+` patterns:
```scala
  SyntacticBlock(init :+ last)
```