Table caption is not parsed the same when using Markdown table or LaTeX table.

[cderv]

Bug description

From https://github.com/quarto-dev/quarto-cli/discussions/1182

---
format: pdf
keep-md: true
keep-tex: true
---

```{r}
#| label: tbl-md
#| tbl-cap: "$\\beta$~1~"
knitr::kable(data.frame(x = rnorm(5)))

#| label: tbl-latex
#| tbl-cap: "$\\beta$~1~"
knitr::kable(data.frame(x = rnorm(5)), format = "latex")

![image](https://user-images.githubusercontent.com/6791940/174641745-ddc6ff1a-56d4-48d1-8e76-a91ddc62462d.png)

In the first case `kable()` will produce Markdown table, and we will process the code in there, setting the table caption as a Caption element in the AST which will preserve the `$ $` math syntax. 
https://github.com/quarto-dev/quarto-cli/blob/0501d0708794a5030fc3b3f64dfca23e93200c0a/src/resources/filters/quarto-pre/table-captions.lua#L136-L140

In the second case, the table produced will be LaTeX, and in this case, we'll process there: 
https://github.com/quarto-dev/quarto-cli/blob/0501d0708794a5030fc3b3f64dfca23e93200c0a/src/resources/filters/quarto-pre/table-captions.lua#L201-L205

We are loosing the `$ $` math syntax when calling `stringify` I believe, so when calling `stringEscape()` the `\beta` will not be correctly parsed as Math. 

If we want to support Markdown syntax for table caption, we should probably align the two treatment. 

Using Dev version of Quarto, on Windows 11.

### Checklist

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[arnaudgallou]

Any chance to have this issue fixed for the v1.3 release? Having this parsing problem breaks everything that comes after (figures, tables and bibliography) and forces you to render figures/tables in individual documents, which is not convenient at all.

[cscheid]

The infrastructure to fix it will be there. But libraries will have to change the way they emit output, so it'll take a while for this to propagate.