ENH: Add Stairs.cummax

[venaturum]

Describe the solution you'd like

See analogous function in pandas https://pandas.pydata.org/docs/reference/api/pandas.Series.cummax.html

Requires skipna parameter. returns new Stairs instance with same initial_value and same value of "closed" property.

Functionality should be straightforward by applying pandas.Series.cummax to Stairs._data["value"] and using Stairs._new

The result will be a monotonically increasing step function

Additional context

https://www.staircase.dev/en/latest/development/guide/internals.html