Make tableEditing function more extendable

[Xheldon]

There is a case that a doc may have two different table type. Thus there should make the tableEditing function which can used by pass a different 'key' to do that:

Plugin({
    ...tableEditing().spec,
    key: new PluginKey('otherSelectionCell'),
})

[Xheldon]

Any ideas about this PR? @RatoX @eshvedai

[Xheldon]

Any ideas about this PR? @RatoX @eshvedai

If this PR not good enough to merge, could you tell me why? Thx in advance!