prefer webshot2 to webshot and solve #1858

[atusy]

With this PR, html_screenshot` tries to use webshot2 and fallbacks to webshot.

Also, user may prioritize webshot by opts_chunk$set(webshot = "webshot").
Another possibility is to use opts_knit$set(webshot = "webshot").
I'm fine with both ways, but choose the opts_chunk because it has controlls on each chunks.

[yihui]

Looks good to me. Just one small question. Thank you!

[yihui]

Great. I'll carry on from here and merge in a few minutes. Thank you!

[yihui]

I forgot to mention that another possible idea is to use pagedown::chrome_print(). We have exported pagedown::find_chrome() there, and it is a CRAN package. I guess the only problem is the compatibility of arguments.

[cderv]

The function are a bit different in their way to find chrome, and specifically with the env var mechanism which is not the same env var. They both will find chrome but there could be a mismatch of version if you have several locally, as webshot2 will use the one that chromote will use.
Also webshot:::find_phantom() was unexported - does feel bad to do the same.

I would stick to the one in chromote for now, and maybe ask them to put one in webshot2 as a helper to test availability of chrome for other package ? or better that they do the check themselves maybe.

[cderv]

find_chrome is now exported from chromote. Not yet from webshot2 I believe.