Usage
.plot_funnel(x, label_es = "Effect Size", conf_level = c(0.95, 0.99),
conf_linetype = c("dashed", "dotted"), conf_fill = NA,
conf_alpha = 1, null_effect = NA, null_conf_level = c(0.9, 0.95,
0.99), null_conf_linetype = c("solid", "dashed", "dotted"),
null_conf_fill = "black", null_conf_alpha = c(0, 0.2, 0.4))