Add dedicated plot.parttree method

[grantmcdermott]

Closes #14. Closes #16. Addresses #9 (even if it doesn't provide a formal fix).

This PR introduces a formal "parttree" class and provides a native plot.parttree method using base R graphics (powered by tinyplot). The ggplot2 scaffolding is still supported via geom_parttree().

Example from the new README:

library(rpart)
library(parttree)

fit = rpart(Kyphosis ~ Start + Age, data = kyphosis)
fit_pt = parttree(fit)
# Default plot
plot(fit_pt)

# Cusomized plot 
plot(
  fit_pt,
  border  = NA, # no partition borders
  pch     = 19, # filled points
  alpha   = 0.6, # point transparency
  grid    = TRUE, # background grid
  palette = "classic", # new colour palette
  xlab    = "Topmost vertebra operated on", # custom x title
  ylab    = "Patient age (months)", # custom y title
  main    = "Tree predictions: Kyphosis recurrence" # custom title
)

^{Created on 2024-07-25 with reprex v2.1.0}

It includes several smaller changes at the same time, e.g. flipaxes is renamed to flip.