Add a new operator "Make Shape Key the Basis Key", shown in the Shape
Key menu as "Make Basis". Effectively this does the same thing as moving
the active shape key to the top of the list of shape keys: applying it
to the mesh.
This operator is added now, because !141930 will prevent moving shape
keys to the top of the list. This hopefully prevents accidentally making
other keys the basis key, which can cause issues that are hard to
recover from.
Pull Request: https://projects.blender.org/blender/blender/pulls/143200
301a86d700