Respect cols_merge() + sub_*() with opt_interactive()

[olivroy]

Summary

Basically, this assumes that the first variable is always where the merge happened? Seems so, based on

Basically, the reason why the merged cells didn't show is because extract_cells() was not run on newly created variavbles.

Note that dt_col_merge_get_vars() seems to properly return NULL when no merge happens!

I added substitution since the fix was very similar!

# Html
gt_tbl <- sp500 |>
    dplyr::slice(50:55) |>
    dplyr::select(-volume, -adj_close) |>
    gt::gt() |>
    cols_merge(
        columns = c(open, close),
        pattern = "{1}&mdash;{2}"
    ) |>
    cols_merge(
        columns = c(low, high),
        pattern = "{1}&mdash;{2}"
    ) |>
    cols_label(
        open = "open/close",
        low = "low/high"
    ) 
gt_tbl

# Before
gt_tbl |> opt_interactive()

# This PR
gt_tbl |> opt_interactive()

Related GitHub Issues and PRs

Fixes #1785, Fixes #1759 Fixes #1846

Checklist

• I understand and agree to the Code of Conduct.
• I have listed any major changes in the NEWS.
• I have added testthat unit tests to tests/testthat for any new functionality.

[rich-iannone]

LGTM!

[rich-iannone]

Thank you @olivroy for getting to the bottom of this and resolving with the PR! Will merge this once CI passes.