[grid-pro] Add cell attribute indicating edit mode

[rolfsmeds]

An attribute on the cell that indicates whether or not the cell is currently in edit mode would be useful when there is a need to apply custom styling to cells in edit/non-edit mode specifically.

[rolfsmeds]

In light of vaadin/platform#3149, editing state should be exposed as a part name on the cell, e.g. cell-editing.