Fix default conceal for xi forms

[gilch]

Currently it's using xⁱ ; x₀ - x₉. The reason for the xⁱ instead of xᵢ is that U+1D62 in unavailable in many programming fonts, though it is in Consolas, for example. So now we're mixing superscripts with subscripts.

A better default might be x¡ with x₀ - x₉. This is "U+001A: Inverted Exclamation Mark". It resembles the desired subscript i pretty closely and it's in pretty much every font.

A possible alternative is all superscripts:
xⁱ ; x⁰ - x⁹.
This kind of looks like a polynomial, so subscripts are better defaults. You could write them in the opposite order to make them look less like exponents:
ⁱx ; ⁰x - ⁹x.

⁴⁵⁶⁷⁸⁹ are U+2074 through U+2079. ²,³, ¹, and ⁰ are U+00B2, U+00B3, U+00B9, and U+2070, respectively.

[farhaven]

I went with the first suggestion, polinomial-style superscripts were just too confusing 😄