Add "open with system default application" event to UI comm

[jennybc]

Addresses #5486
Companion PR in ark: posit-dev/ark#877

This PR creates a new event in the UI comm that says "open this thing (file, presumably) with the default application according to the OS".

The immediate motivating example is to allow R's browseURL() to work as expected in the Positron console.

Release Notes

New Features

• browseURL() in R now delegates to the operating system's default opener for inputs that are not recognized as a web URL or an HTML file.

Bug Fixes

• N/A

QA Notes

In R, use browseURL() on a local filepath. Exercise a couple of files (meaning file types) and a folder. Here are examples taken from the "R Code" column of the table at #5486 (comment):

R Code	Expectation in Positron R Console, with this PR + ark PR
browseURL(tempdir())	opens folder in OS's notion of file explorer (e.g. Finder on macOS)
temp_csv_file <- tempfile()  write.csv(iris, temp_csv_file)  browseURL(temp_csv_file)	opens the csv file in OS's default application (for text files? on macOS, I'm getting TextEdit)
temp_csv_file_with_ext <- tempfile(fileext = ".csv")  write.csv(iris, temp_csv_file_with_ext)  browseURL(temp_csv_file_with_ext)	opens the csv file in OS's default application for .csv (VS Code for me, haha)
temp_html_file <- tempfile()  writeLines("<p>Hi!</p>", temp_html_file)  browseURL(temp_html_file)	opens the html file in OS's default application (for text files? on macOS, I'm getting TextEdit)
temp_html_file_with_ext <- tempfile(fileext = ".html")  writeLines("<p>Hi!</p>", temp_html_file_with_ext)  browseURL(temp_html_file_with_ext)	opens .html file in default external browser

When in doubt, the behaviour we seek is whatever happens in R in a plain terminal or in RStudio Console.

We should definitely verify this on linux and Windows (I'm on macOS, so feel pretty confident about that).

As for tests, I don't really know how we would do that 🤔 in this case. The whole point is to open a file in some other application and exactly what that is will vary by OS / user.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

^{readme  valid tags}

[jmcphers]

LGTM, thank you for adding this!