Add support for webr kernel

[ryanlovett]

There is a very new kernel that can run R in the browser, https://github.com/r-wasm/jupyterlite-webr-kernel. It'd be great to showcase it at try.jupyter.org.

[jtpio]

Thanks @ryanlovett for the suggestion.

This R kernel looks great. However their README says:

In order to use the webR kernel with JupyterLite the page must be served with certain security-related HTTP headers

Since we don't control the headers when using GitHub Pages, not sure the kernel will be able to work properly.

We can still open a draft PR and try test the kernel.

[manics]

@choldgraf if the R kernel works with JupyterLite but not on GitHub pages do you think try-jupyter could be hosted on infrastructure owned by mybinder? I think the upfront cost would be the effort to setup an automated deployment for a static site with custom headers. This shouldn't require a server, so the ongoing cost would be for bandwidth (how much?).