A HTML widget viewer for the webR application, similar to browseURL()

[georgestagg]

A function webr::viewer_install() is added to the webR support package. The function sets R up so as to generate an output message over the webR communication channel when a URL viewer is invoked using options("viewer").

Printing a HTML element or HTML widget in the webR application app now shows the resulting HTML output in an embedded viewer iframe, meaning packages like gt() can now work.

c.f. #295, #384, #431.

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Over the months I have considered several ways to handle requests to browseURL() from webR. After pondering for enough, in the interests of pragmatism this PR contains a "first-pass" approach that replaces relative JS and CSS sources in the output HTML file with content directly (encoded in base64) then displays the result in an iframe. With thanks to @timelyportfolio, as this is an improvement to their basic implementation method outlined here.

In the future, one or more of the following should be done, in order of "correctness":

• Other types of relative URL in the output should be found and replaced.
• Proper XML parsing should be done on the output, not regex replacement.
• The output should instead be made self-contained in a more robost way, e.g. using Pandoc compiled for wasm.
• The R object should instead be send to the main thread, not the HTML output. That way adding HTML dependencies to the page can be handled better in JavaScript.
• A JS Service Worker should be started, serving the /tmp directory containing the HTML and support files. With this, relative URLs would "just work" without any modification.

[timelyportfolio]

@georgestagg Wow, this is absolutely amazing. I'll dive deeply over the week keeping in mind the list of "to-do". Let me know if there is anything I can do to help.