\QuartoMarkdownBase64 command in LaTeX formats

[cscheid]

In LaTeX raw blocks and LaTeX output formats, \QuartoMarkdownBase64{<<base64content>>} works in LaTeX analogously to how <span data-qmd-base64="<<base64content>>" works in all quarto formats.

Specifically, this allows third-party-libraries that emit LaTeX programmatically to "inject" quarto-compatible markdown in their RawBlocks.

This is necessary for the same reason that the <span> trick is necessary. On occasion, a single RawBlock is interpreted (and its contents are parsed) as a full unit. Currently, we parse tables in HTML rawblocks, and do table fixups in LaTeX rawblocks. Both of these require a "single" RawBlock to work.

Here's a minimal example:

---
format: pdf
a_meta_key: a_value
---

![A caption](./foo.jpg){#fig-1}

```{=latex}
% QGZpZy0x is "@fig-1" in base-64 encoding
% U29tZSBfbWFya2Rvd25fIGhlcmUu is "{{< meta a_meta_key >}} Some _markdown_ here." in base-64 encoding
\begin{tabular}{|c|c|}
col1 & col2 \\
\QuartoMarkdownBase64{QGZpZy0x} & \QuartoMarkdownBase64{e3s8IG1ldGEgYV9tZXRhX2tleSA+fX0gU29tZSBfbWFya2Rvd25fIGhlcmUu} \\
\end{tabular}
```

This renders to:

Notice that both crossrefs and shortcodes are resolved properly.

It might be tempting to think that this feature isn't necessary, and to try something like

```{=latex}
\begin{tabular}{|c|c|}
col1 & col2 \\
```
@fig-1 ` & `{=latex} {{< meta a_meta_key >}} Some _markdown_ here. `{=latex} \\`
```{=latex}
\end{tabular}
```

The problem is that the matching pair of \{begin,end}{tabular} commands are now broken across different RawBlocks, and quarto is then incapable of applying some fixups.

We purposefully don't offer a non-base-64 encoded version because parsing the valid non-latex content inside the LaTeX rawblock would be annoyingly hard to do well. We expect this feature to be used by third-party libraries like gt essentially all of the time.

[cscheid]

@dragonstyle This adds an explicit API for something similar to what you call the "markdown pipeline". At some point we should figure out if there's a way to consolidate the two paths. They're not quite the same but I wanted to point this out anyway.

[cderv]

So cool ! Thanks !