[Feature request] Add option to define code button alignment

Many thanks for creating and maintaining this package!

Would it be possible to add an option that defines where on the page the "Show code" button will be located if using the option "code-fold: true" when rendering HTML? I find that having the code button on the left breaks my reading flow, and I would prefer if the button was located in the center or on the right of the page.

I would propose to add an option "code-align" with can take the values "left", "center", or "right" (as illustrated below).

Apologies if this feature already exists, I couldn't find it in the documentation.

---
title: "Title"
format:
  html:
    code-fold: true
    code-summary: "Show code"
    code-align: "right" # Proposed options: "left", "center", "right"
---

```{r}
library("ggplot2")
dat <- data.frame(cond = rep(c("A", "B"), each=10),
                  xvar = 1:20 + rnorm(20,sd=3),
                  yvar = 1:20 + rnorm(20,sd=3))

ggplot(dat, aes(x=xvar, y=yvar)) +
    geom_point(shape=1) + 
    geom_smooth() 
```

The issue does not name implementation files or tests. Start by locating the HTML code-fold "Show code" button and the handling of the code-fold and code-summary options, then trace how its alignment is styled. Done means a documented code-align option supports left, center, and right without changing the existing defaults.