Improve stub with indentation options and better default alignments

[rich-iannone]

This PR focuses on some first steps to improving the appearance and functionality of the gt table stub.

The first fixes (a prelude, really) involve improving the default alignments of content in the stub. With the stub serving either as row labels or indices, it's important to discern which is which and this is done as part of the auto-alignment routine in the gt() function.

The main thing after that is to introduce the first-class ability to set indentation on the stub elements. This is important enough to require a new function with a specialized interface for this task (tab_stub_indent()).

Here is a runnable example that demonstrates one use of tab_stub_indent():

library(gt)
library(tidyverse)

exibble %>%
  select(-c(fctr, date, time, datetime)) %>%
  gt(rowname_col = "row", groupname_col = "group") %>%
  sub_missing() %>%
  summary_rows(
    groups = TRUE,
    columns = num,
    fns = list(
      average = "mean",
      total = "sum",
      SD = "sd"
    )
  ) %>%
  tab_options(row_group.as_column = TRUE) %>%
  tab_stub_indent(indent = 2, rows = matches("2|3|6"))

[cscheid]

Is this the kind of thing that's best done by adding a new concept ("indent") or should it be left to user CSS instead?

[rich-iannone]

IMO it’s far better to make this first class with its own function since it’s meant to establish structure and the implementation is different across output types.

[cscheid]

lgtm.