Make a sd_is_answered() function

[jhelvy]

It would be really helpful if we could determine whether a question was answered or not. This function would just return TRUE or FALSE depending on whether a user has clicked on a choice or typed some text, etc.

The use case I imagine for this would be inside sd_show_if or sd_skip_if where a user wants a condition on whether the conditional question has been answered at all or not. The only way to do this right now is to include a full set of conditions for all options, which is tedious. I'm imagining the use case like this:

sd_show_if(
    sd_is_answered(input$question_id) ~ "target_question_id"
)

A special consideration of this is matrix type questions, as mentioned in #105 here. We should be able to tell if any one row in a matrix type question is answered as well as if all rows are answered. For example:

sd_question(
  type   = "matrix",
  id     = "car_preference",
  label  = "Please indicate your agreement with the following statements.",
  row    = c(
    "I'd like to buy a gasoline car." = "buy_gasoline",
    "I'd like to buy an EV." = "buy_ev"
  ),
  option = c(
    "Disagree" = "disagree",
    "Neutral"  = "neutral",
    "Agree"    = "agree"
  )
)

For the above matrix type question, we should be able to condition on a single row like this:

sd_show_if(
    sd_is_answered(input$car_preference_buy_gasoline) ~ "target_question_id"
)

And also on the entire matrix like this (here this would be TRUE only if all rows are answered:

sd_show_if(
    sd_is_answered(input$car_preference) ~ "target_question_id"
)

[jhelvy]

This is now complete as of v0.3.7