Make Math digits tabular

[RuixiZhang42]

I was surprised to find out that in the STIX Two Math font, the digits were not tabular! This is not good for alignment in a system of equations.

Here are some suggestions:

1. The regular (upright) digits should be made tabular in all 3 optical variants, at least as the default.
2. The tabular digits in the different optical styles (text, script, and scriptscript) can have different advance widths. Maintaining one width within an optical style is enough.
3. Whether the bold digits (U+1D7CE to U+1D7D7) should be duplexed with the regular digits (U+0030 to U+0039) across all 3 optical styles is open to discussion.
4. Whether the sans-serif digits (U+1D7E2 to U+1D7EB) should be tabular is open to discussion.
5. Whether the sans-serif bold digits (U+1D7EC to U+1D7F5) should be duplexed with the sans-serif digits (U+1D7E2 to U+1D7EB) across all 3 optical styles is open to discussion.
6. Maybe the double-struck/blackboard digits (U+1D7D8 to U+1D7E1) are better off proportional. But again, this is also open to discussion.

[RuixiZhang42]

• I forgot to include the slashed zero (U+0030 followed by U+FE00) in the picture, sorry! It also belongs to the regular upright digit group (and hopefully made tabular, too).
• Only the regular digits (plus slashed zero) have optical variants. Each plane-1 digit has just one text-size glyph, no small-size variants. So I guess duplexing can only occur between regular and bold in the text style?
• The sans-serif digits and the sans-serif bold digits are already tabular, just not duplexed.