[Noto Sans Mono] Unslashed zero stylistic set

[joncar]

I was trying to print Noto Sans Mono at 6pt and the slashed zero loses its benefit (in fact, it becomes harder to read). This adds a zero.noslash glyph which is a copy of the zero glyph with the slash removed. It is accessed via a new stylistic set ss01 ("Unslashed zero").

I removed the do-nothing zero tag since it replaces the slashed zero ... with the slashed zero.

Resolves issue #167.

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[simoncozens]

Thank you!