Feature request: programmatic value box styling for OJS dashboards

Currently, value boxes in Quarto dashboards can be programmatically customized in R and Python, as in this example from the docs:

```{python}
#| content: valuebox
#| title: "Comments per day"
dict(
  icon = "chat",
  color = "primary",
  value = comments
)
```

```{r}
#| content: valuebox
#| title: "Spam per day"
list(
  icon = "trash",
  color = "danger",
  value = spam
)
```

The OJS engine works really well with Quarto dashboards, as it allows for interactivity without a server and with much better responsiveness than Shiny or Shinylive.

It would be great if we could do something like the following in dashboards and have the same output as above:

```{ojs}
//| content: valuebox
//| title: "Comments per day"
({
  icon: "chat",
  color: "primary",
  value: comments
})
```

The parentheses are needed for OJS. Here, comments would be another variable defined in another OJS cell or from R/Python with ojs_define.

The issue names no source files or tests. Start by tracing how Quarto dashboard value boxes consume R/Python cell results and how OJS cells expose values via ojs_define; done means an OJS object with icon, color, and value renders the same value box output.