Add support for ggcoefplot

[grantmcdermott]

This PR adds (gg)coefplot support alongside the existing (gg)iplot functionality.

Quick examples from updated help docs.

pkgload::load_all("~/Documents/Projects/ggiplot/")
#> ℹ Loading ggiplot
#> Loading required package: ggplot2
library(fixest)

##
# basic use

est = feols(
    Petal.Length ~ Petal.Width + Sepal.Length + Sepal.Width + Species,
    data = iris
    )

ggcoefplot(est)

# You can use the argument group to group variables
# You can further use the special character "^^" to clean
#  the beginning of the coef. name: particularly useful for factors

# now we group by Sepal and Species
ggcoefplot(est, group = list(Sepal = "Sepal", Species = "Species"))

# now we group + clean the beginning of the names using the special character ^^
ggcoefplot(est, group = list(Sepal = "^^Sepal.", Species = "^^Species"))

##
# comparison of ggcoefplot and ggiplot

data(base_did)
base_inter = base_did

# We interact the variable 'period' with the variable 'treat'
est_did = feols(y ~ x1 + i(period, treat, 5) | id + period, base_inter)

# In the estimation, the variable treat is interacted
#  with each value of period but 5, set as a reference

# ggcoefplot will show all the coefficients:
ggcoefplot(est_did)

# ggiplot just shows the interactions
ggiplot(est_did)

^{Created on 2023-12-03 with reprex v2.0.2}