Prepend a Vector to a NonEmptyVector (#3362)

* Add prependVec

I needed to implement this recently for some work
I was doing and was surprised it didn't already exist.

* Run +prePR

* Fix according to PR recommendations
- Use idiomatic implementation
- Use more consistent naming

core/src/main/scala/cats/data/NonEmptyVector.scala

@@ -115,6 +115,12 @@ final class NonEmptyVector[+A] private (val toVector: Vector[A])
    */
   def prepend[AA >: A](a: AA): NonEmptyVector[AA] = new NonEmptyVector(a +: toVector)
 
+  /**
+   * Prepend a `Vector` to this, producing a new `NonEmptyVector`.
+   */
+  def prependVector[AA >: A](vector: Vector[AA]): NonEmptyVector[AA] =
+    new NonEmptyVector(vector ++ this.toVector)
+
   /**
    * Alias for [[prepend]]
    */

tests/src/test/scala/cats/tests/NonEmptyVectorSuite.scala

@@ -247,6 +247,18 @@ class NonEmptyVectorSuite extends NonEmptyCollectionSuite[Vector, NonEmptyVector
     }
   }
 
+  test("prependVector with a NonEmptyVector is the same as concatNec") {
+    forAll { (nonEmptyVector1: NonEmptyVector[Int], nonEmptyVector2: NonEmptyVector[Int]) =>
+      nonEmptyVector2.prependVector(nonEmptyVector1.toVector) should ===(nonEmptyVector1.concatNev(nonEmptyVector2))
+    }
+  }
+
+  test("prependVector with an empty Vector is the same as the original NonEmptyVector") {
+    forAll { (nonEmptyVector: NonEmptyVector[Int]) =>
+      nonEmptyVector.prependVector(Vector.empty) should ===(nonEmptyVector)
+    }
+  }
+
   test("NonEmptyVector#of on varargs is consistent with NonEmptyVector#apply on Vector") {
     forAll { (head: Int, tail: Vector[Int]) =>
       NonEmptyVector.of(head, tail: _*) should ===(NonEmptyVector(head, tail))