[Feature Request]: Subset `teal_card` elements when subsetting `teal_report`

[averissimo]

Feature description

When subsetting a teal_report object with [.teal_report function, the cards should also be subsetted.

There are some questions to solve as there is no 1-1 relationship between code that is subsetted and cards (there might be text/outputs/....)

Topics to discuss:

• Should we keep manual cards & outputs (@m7pr thinks so)
  • Alternative: generate teal_cards from code (maybe using .code_to_card)?

Mock tests below:

test_that("Subsetting teal_report also subsets teal_cards", {
  q <- within(teal_report(), {
    data1 <- iris
    data2 <- mtcars
    summary(data1)
    summary(data2)
  })

  q2 <- q["data1"]
  expect_equal(
    teal_card(q2),
    teal_card(q)[c(1,3, 4)] 
    # Q: should we keep the output of the code
  )
})

test_that("Subsetting teal_report also subsets teal_cards", {
  q <- within(teal_report(), {
    data1 <- iris
    data2 <- mtcars
    summary(iris)
    summary(mtcars)
  })

  teal_card(q) <- c(teal_card(q), "My first manual card")

  q2 <- q["data1"]
  # Q: should we keep the manual cards
})

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