Better handle heading alignment in LaTeX output

[rich-iannone]

In LaTeX output, very long titles/subtitles will trigger a shift in alignment from centered to left-aligned. This PR makes the alignment explicit, based on the chosen option (from tab_options(heading.align = ...)) or the "center" default.

Fixes: #630

[cscheid]

I don't think this one is quite right. If no heading_align is present, we should instead not add any \captionsetup at all, so that the latex template defaults take over.

[rich-iannone]

By default there is a heading_align present and its value is "center":

gt/R/dt_options.R

Line 72 in 88dacad

"heading_align", TRUE, "heading", "value", "center",

Before this PR, there was no checking of this property value at all so the default caption justification was always used. I like the idea of the default caption being an option but I'm not sure at this point how to incorporate that option.

[cscheid]

But the problem is that if the user hasn't set this explicitly, GT should let the styling be dictated by the LaTeX template. It shouldn't be up to your defaults: it should be up to the user's template. (this is the analogous LaTeX discussion to our HTML styling discussion yesterday.)

I understand that it is the current GT behavior to set these visual attributes. But in formats such as LaTeX (and themed HTML outputs from quarto as well), the environment should provide the appearance defaults.

[rich-iannone]

Ah, that's right, that should apply here as well. I'll do that work in a separate PR and then revisit this one once we can distinguish between user set and unset property values.