[add] exponent modes

[Mc-Zen]

This PR introduce a settable parameter num.exponent that can be set to

• auto (default behaviour, displays the explicitly given exponent),
• "sci" (employ scientific notation), or (sci: n) to activate scientific notation only when the absolute of the exponent is at least n or (sci: (min, max)) to activate scientific notation only when the exponent is less or equal to min or greater or equal to max
• "eng" (employ engineering notation),
• (fixed: n) to fix the exponent.

The latter three options are regardless of the input exponent, the number is just updated accordingly and the input exponent is taken into account.

Moreover, a new parameter prefix: auto | none has been added to the unit settings. It can be set through set-unit(prefix) or when calling a unit zi.m(prefix: auto)[2km]. When the prefix is set to auto and num.exponent is set to "eng" an automatic prefix will be added to the unit, replacing the exponent. This feature is by default activated.

#set-num(exponent: "eng")

#zi.s[1e-7] // will produce "100ns"

Closes #17, closes #28.

[amesgen]

sci does exactly what I wanted, thanks a lot!