i'd like to manually create axis origin symbol for `school-book` axis style

[johannes-wolf]

Discussed in #150

^{Originally posted by xianxingg October 3, 2025}
for now, it's hard-coded in plot.typ:

if axis-style == "school-book" { axes.school-book(size: size, axis-dict.x, axis-dict.y) }, this makes shared-zero always set to true inside school-book function, and it renders 0

sometimes this 0 is crossed by lines (and i want to adjust 0's position), sometimes i don't need 0 but big O

i'd like to use cetz-plot in this way: plot(..., axis-style: "school-book", shared-zero: false)