Add live updates for the R data explorer

[jmcphers]

This change adds live updating to the R data explorer backend (like Python already has).

Screen.Recording.2024-03-29.at.10.49.35.AM.mov

Addresses posit-dev/positron#2386.

The approach is pretty simple:

• Every time a console prompt appears, we check the data set for updates. We consider an update to have occurred whenever the underlying S-expression changes.
• When an update occurs, we regenerate the column schemas. If they match the old schemas, it's just a data update; if they don't, it's a full schema update.
• Column schema requests are now served from a cache; that cache is generated once when the data viewer starts and again when changes are detected.

Note that we only support live updates for cases where we know name of the variable containing the data and the environment it lives in. Also note that we don't gracefully handle the case where a variable is replaced with something that is not a data frame or matrix; in this case we need to close the comm. That's tracked in posit-dev/positron#2333.

I've also added some integration tests that run against the new functionality.