Add CustomJSTicker

[bryevdv]

The asymmetry of having a CustomJSTIckFormatter while not offering a corresponding CustomJSTicker has low-level bothered me for years. However the utility of having one has come up in a few different support questions latetly, so we should go ahead and add one.

It should be fairly straightforward to adapt existing "CustomJS" code to be callable by Ticker.get_ticks (keeping in mind @mattpap current work to afford imports inside CustomJS code). The main questions to decide:

Ticker.get_ticks expects to return both major and minor tick locations:

export type TickSpec<T> = {
  major: T[]
  minor: T[]
}

Do we want to expose this fully to CustomJS code? Or just expect users to return major ticks? Or have some simpler path to return only major ticks in the common case?

Additionally get_ticks accepts a number of parameters:

get_ticks(data_low: number, data_high: number, range: Range, cross_loc: number):

Do we expose all of these to the CustomJS code or leave off some of the more esoteric ones like cross_loc. In any case how do we pass the data to user code, maybe cb_data?

cc @bokeh/dev