Expose function Html.to_plain_text

[cuihtlauac]

In order to add id attributes, ocaml/ocaml.org has duplicated the function Html.to_plain_text, see: https://github.com/ocaml/ocaml.org/blob/main/tool/ood-gen/lib/tutorial.ml#L28

Therefore, it seems to makes sense to expose this function publicly.