implement rstudioapi::showPrompt() and rstudioapi::askForPassword()

[idavydov]

This would allow keybindings requiring prompts, like:

        {
          "command":"r.runCommandWithSelectionOrWord",
          "args": "dplyr::count($$, .data[rstudioapi::showPrompt("dplyr::count()", "column?")])"
        }

or if we want to support multiple columns:

        {
          "command":"r.runCommandWithSelectionOrWord",
          "args": "dplyr::count(df, !!!rlang::syms(strsplit(rstudioapi::showPrompt("dplyr::count()", "columns?"), "\\s*,\\s*")[[1]]))"
        }