Create Web Utilities vignette

[shikokuchuo]

Take the ncurl (HTTP) and stream (WebSocket) sections from the existing vignette in dev/vignettes and create a new file _web.Rmd.

You will need to modify dev/vignettes/precompile.R to add the new file, and run the command to pre-compute the vignette to the vignettes folder.

• As this is a separate vignette, there is room to expand it to include example usage of ncurl(), ncurl_aio() and ncurl_session().
• Add an example of ncurl_aio() being used within a Shiny ExtendedTask.