Export `topic_write_html()`

[lionel-]

Used in RStudio already: https://github.com/rstudio/rstudio/blob/4a20e65e129178ecadcffc53b7ad48b2c9781769/src/cpp/session/modules/SessionHelp.R#L781-L792

[DavisVaughan]

Might make sense to export it along with topic_write_text() jointly as topic_write(type = c("text", "html"))

(but we'd have to keep the underlying topic_write_html() in place for a long time for RStudio, probably worth adding a comment in the code about that)