[FR] Support TOC depth customization in `bs4_book`

[cderv]

Two issue has been opened on this already

• TOC depth int bs4_book #1377
• [FR] bs4_book: toc_depth specification in sidebar #1254

TOC in bs4_book() is not either opt-out or allowing customization. Max level of TOC is h3

bookdown/R/bs4_book.R

Lines 134 to 138 in 7ce6dc6

	build_toc <- function(output) {
	html <- xml2::read_html(output)
	main <- xml2::xml_find_first(html, ".//main")
	headings <- xml2::xml_find_all(main, ".//h1|.//h2|.//h3")

This is by design so that we get a nice styling to the specific TOC (it is not the same toc float as in rmarkdown::html_document)
Doing more levels would require probably some adjustment on styling, and allowing any number of levels does not seem a good choice as the TOC would be cluttered.

Opening this issue to track interest by vote with 👍 for TOC depth customization in bs4_book as it can be reconsidered.