fix: Do not change the default dimensions of altair charts unless they are set with the altair specific options

[joelostblom]

close #1642

This will avoid using the knitr default dimensions from other figure formats for altair charts. I think this makes sense since altair already has sensible and easily overidable defaults. The altair-specific kntir options are left as before and can still be used (I think it would also be an option to scrap these because as noted in the source comments, they don't work for compound charts whereas the settings in altair does).

The behavior this PR introduces is consistent with how altair works elsewhere, such as in web document and IDEs (I could not test in RStudio due to altair charts not being supported there yet as per rstudio/rstudio#8762 and rstudio/rstudio#4259). I believe this would be more intuitive for users than having the default altair dimensions overwritten by knitr (e.g. altair uses adaptive dimensions for categorical data depending on the number of categories and a consistent mark size rather than scaling the marks based on the number of categories, see example with the bar charts below).

Using the following quarto/rmarkdown source doc for testing purposes:

---
format: html
engine: knitr
---

```{python}
import altair as alt

alt.Chart().mark_point()
```

```{python}
import pandas as pd
source = pd.DataFrame({
    'a': ['A', 'B', 'C'],
    'b': [28, 55, 43]
})

alt.Chart(source).mark_point().encode(
    x='b',
)
```

```{python}

source = pd.DataFrame({
    'a': ['A', 'B', 'C'],
    'b': [28, 55, 43]
})

alt.Chart(source).mark_bar().encode(
    x='b',
    y='a'
)
```

```{python, altair.fig.height=200, altair.fig.width=200}

source = pd.DataFrame({
    'a': ['A', 'B', 'C'],
    'b': [28, 55, 43]
})

alt.Chart(source).mark_bar().encode(
    x='b',
    y='a'
)
```

Quarto output BEFORE this PR

Quarto output AFTER this PR

RMarkdown output AFTER this PR

The behavior before and after is the same as for quarto so just showing one screenshot here indicating that things are working as expected.

[joelostblom]

I am not sure if this line (and the corresponding one for height) are still needed, but I left them in for now.

[t-kalinowski]

Thank you for the PR! This seems good to me, but I'm a bit removed from altair charts and don't know how impactful or breaking this change will be for users who are currently expecting the default knitr chunk option to be respected.

@cderv can you weight in? I'm inclined to merge.

[t-kalinowski]

I'll go ahead and merge. @joelostblom can you please add a NEWs entry?

[joelostblom]

Added, thank you @t-kalinowski !

[t-kalinowski]

Thank you!