[WIP] Allow major/minor units

[JanMarvin]

idea for major/minor units without API changes
For #90 and #105

[davidgohel]

Thanks for this PR, it works well

I'd like to suggest a slightly different API approach. Instead of encoding the unit value in the name of the major_tick_mark/minor_tick_mark vector, I could add dedicated major_unit and minor_unit parameters:

chart_ax_x(x, 
  major_tick_mark = "cross",
  major_unit = 0.5, # default to NULL
  minor_tick_mark = "none",
  minor_unit = 0.1 # default to NULL
)

This keeps the tick mark type ("in", "out", "cross", "none") and the unit interval as two separate concerns, which feels more conventional as an API.

What do you think?

[JanMarvin]

Perfect, I just didn't want to introduce a break.
Maybe chart_ax_x() and chart_ax_y() could be combined (I only had a quick look, but they both seem to share the same code). Let me know if you want to take this over or if I should give it a try.