R: Extended LSP requests for object at cursor

Originally posted by @lionel- in #2974:

Relatedly, I've been thinking for a while about porting a feature I created in Emacs and used all the time as a data analyst. This feature has two parts:

• Detect relevant object at cursor. For instance, foo$bar$q<>ux gives you foo$bar$qux, foo$bar<>$qux gives you foo$bar, and f<>oo$bar$qux gives you foo. This would be provided via an extended LSP request.

• Provide various exploration commands for the object at cursor: print(), summary(), str(), lobstr::sxp(), etc. These exploration commands should be user-configurable (simple list of strings with some kind of symbol representing where the input goes). And View() could be one of the default actions.

The commands could each be an assist but I'd worry about cluttering the assist menu and making it harder to use as we add new kinds of assists in the future. So we could have a single assist for "Actions at point" that opens up a quickpick menu. It'd be great to implement frecency (thanks for the notion @DavisVaughan!) sorting of these commands.

This should also be its own command to make it quicker to use (no need to open the assist menu). Making it an assist is good for discoverability though. And there should also be a command to directly run an action. The command would take an action string as argument. This could be bound to Cmd+Click with View(%%) as action for instance.

No files or tests are named. Start by locating the existing LSP, assist, and command entry points; use the cursor examples to define the object request and action flow, then verify configurable actions, View(), direct invocation, discoverability, and frecency behavior.