Introduce the from_column() helper function and implement its use in tab_stub_indent()

[rich-iannone]

This PR introduces the from_column() helper function, which is meant to help define a set of values for a compatible parameter (argument). In this first part of the work, the helper is defined with some basic arguments of its own; the first working implementation of it is with the tab_stub_indent() function.

Here is an example of how it works in practice, where we get row-by-row values for the indent parameter from the rank column.

exibble |>
  dplyr::select(char, fctr, currency) |>
  dplyr::mutate(rank = dense_rank(desc(currency))) |>
  dplyr::arrange(rank) |>
  gt(rowname_col = "char") |>
  tab_stub_indent(
    rows = everything(),
    indent = from_column(column = "rank", na_value = 3, fn = NULL)
  )

Notice that the row labels in the stub are indented according to the values in the rank column.

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