Improve as_gtable() width calculations

[teunbrand]

Summary

This PR aims to fix #1923.

I've tried to mirror to CSS specifications more closely with the constraint that grid can't automatically reflow text.
See examples below.

Related GitHub Issues and PRs

• Ref: as_gtable not respecting cols_width in as_gtable when tab_options(table.width = pct(100))? #1923
• Test change: Pct column widths #1776

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.

Examples

Based on the reprex in the linked issue, showing the plot with no total width contraints, a relative width and an absolute width contraint.

devtools::load_all("~/packages/gt/")
#> ℹ Loading gt
library(gtExtras)
library(tidyverse)

start_date <- "2010-06-07"
end_date <- "2010-06-08"

sp_tab <- 
  sp500 %>%
  select(-c(volume, adj_close)) %>%
  dplyr::filter(date >= start_date & date <= end_date) %>%
  gt() %>%
  cols_width(date ~ pct(50)) %>%
  tab_options(table.font.size = px(12))

# No total width contraints  
plot(sp_tab)

plot(sp_tab %>% tab_options(table.width = pct(80)))

plot(sp_tab %>% tab_options(table.width = px(500)))

^{Created on 2024-12-09 with reprex v2.1.1}

[teunbrand]

This expectation was introduced in #1776 and had to change, please let me illustrate why.
The HTML has a tiny bit of whitespace to the left of 'x' and plenty to the left of 'y'.

library(gt)

tbl <- data.frame(x = 1, y = 1) |>
  gt() |>
  cols_width(x ~ pct(20), y ~ px(200))

gtExtras::gt_reprex_image(tbl)

Currently, too much space is allocated to the left of 'x'

plot(tbl)

With this PR, we more closely mirror the HTML output:

devtools::load_all("~/packages/gt/")
#> ℹ Loading gt
plot(tbl)

^{Created on 2024-12-09 with reprex v2.1.1}

[rich-iannone]

LGTM, thanks!