New stat: stat_connect()

[teunbrand]

This PR aims to fix #6228 and replaces #6231.

It follows the advice in #6231 (comment), where instead of implementing the idea as a position adjustment, it is implemented as a stat. Together with #6326, this should offer greater flexibility than a position adjustment.

The example section rendered:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(head(economics, 20), aes(date, unemploy)) +
  stat_connect(connection = "hv")

# Setup custom connections
x <- seq(0, 1, length.out = 20)[-1]
smooth <- cbind(x, scales::rescale(1 / (1 + exp(-(x * 10 - 5)))))
zigzag <- cbind(c(0.4, 0.6, 1), c(0.75, 0.25, 1))

ggplot(head(economics, 10), aes(date, unemploy)) +
  geom_point() +
  stat_connect(aes(colour = "zigzag"), connection = zigzag) +
  stat_connect(aes(colour = "smooth"), connection = smooth)

^{Created on 2025-02-12 with reprex v2.1.1}