Add basic support for specifying column widths in LaTeX table outputs

[rich-iannone]

This adds rudimentary support for defining column widths for LaTeX output tables. Accepts length values in px which, in LaTeX, is equivalent to bp (big point, 1/72 in; 1 pt almost equivalent to 1 bp) units.

Here is a working example table that defines widths for the stub's rowname column, and two columns in the table body. It is okay to define only a few column widths.

dplyr::tibble(
  row = "one",
  group = "a",
  a = "The development of industrial typefaces began in the late 19th century and was greatly influenced by the industrial and technological advancements of the time. Industrial typefaces are distinguished by their strong sans serif letterforms, straightforward appearance, and use of geometric shapes and straight lines.",
  b = 23.6
) |>
  gt(rowname_col = "row", groupname_col = "group", row_group_as_column = TRUE) |>
  cols_width(row ~ px(80), a ~ px(150), b ~ px(100))

Fixes: #851
Fixes: #634

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