Tolerate strings without units for margins

[markcharyk]

Right now, setting margins in a section works as intended when passed number values:

page: {
    margin: {
        top: 1134,
        right: 1134,
        bottom: 1134,
        left: 1418,
    },
},

However, each property in the margins hash is typed as number | string | undefined which leads me to believe that the following should also work:

page: {
    margin: {
        top: "1134",
        right: "1134",
        bottom: "1134",
        left: "1418",
    },
},

This, however, throws an error, because string values are expected to have units specified (see this utility function).

I think the experience would be improved by tolerating strings containing un-united numbers for specifying an integer number of twips.