Blender has a couple basic wipes to transition between shots. But with the addition of the new Mask based Strip Modifier, you can define a transition using any shape you want! Create a black and white gradient image (or high contrast if you prefer) and use that as a mask source (or matte if you’re old like me). Then change the ‘blend’ type of the B strip (incoming strip) to alpha over and apply a Mask Modifier that uses the imported artwork.

You can animate the contrast of the source art (matte) to alter over time, or you could create an actual animation to use instead.

Here is a tutorial describing the workflow.
DIY wipe tutorial