Include library() and require() shims in webr::shim_install()

[georgestagg]

Shim base R library() and require() commands so that webR packages are downloaded automatically with an optional menu.

It is no longer assumed that an interactive R session will be able to show a menu() when offering to download a missing R package. Instead, this now defaults to not showing a menu, with options(webr.show_menu = TRUE) enabling the menu feature globally.

This extends the functionality of the global packageNotFoundError handler so that the same feature can be used when using evalR(). The new shims also ensure that library() and require() do not need to be called again once the package is downloaded and available.

I have retained the original global handler solution as it provides additional benefits over simply shimming library() and require(). For example, when the packageNotFoundError handler is active packages can be automatically installed on demand if functions are referred to via namespace, such as cli::cli_alert_success('The cli package works!').

The webR REPL application now enables both features, including prompting with the confirmation menu, by default.

Fixes #324, #325.