Enhance the extract_body() function by adding inclusion options

[rich-iannone]

This update of the extract_body() function includes two new arguments: incl_hidden_cols and incl_stub_cols. Previously, the function always returned a data frame that contained hidden columns. And, though stub columns were included, there was no way to exclude them from the returned data frame.

Fixes: #1875

[olivroy]

I agree with the changes! Maybe it would be worth adding ... + rlang::check_dots_empty() to put output as last argument and force naming the argument explicitly to minimize breaking changes in the future.

Since this is technically a breaking change, it is worth adding to news.

gt_tbl <- gt(exibble)
# before this PR, this would work.
extract_body(gt_tbl, "unfmt_included", "html")

[rich-iannone]

Thanks for the review! I'll add a note to NEWS describing the breaking changes here. Also, before finishing this up, I'll add 1-2 examples since there aren't any right now.