Add zero.slashed

[MDLC01]

This assigns the name zero.slashed to the DIGIT ZERO short diagonal stroke form variation sequence.

zero alone is mapped to a regular 0 with no variation selector with a text variation selector.

[T0mstone]

Is there a reason for the text variation selector?

[MDLC01]

Because a version with the emoji variation selector exists, and symbols in sym that have both a text presentation and an emoji presentation always specify the text presentation explicitly. See #114 for more context.

[T0mstone]

Ah, right, thanks.