[feature request] add support for png export

[OrangeX4]

The code mentions that you can export pdf, svg, and png formats:

typst-py/python/typst/__init__.pyi

Lines 28 to 31 in d1c177c

	Args:
	input (PathLike): Project's main .typ file.
	output (Optional[PathLike], optional): Path to save the compiled file.
	Allowed extensions are `.pdf`, `.svg` and `.png`

But actually we can only export pdf:

typst-py/src/compiler.rs

Lines 27 to 30 in d1c177c

	match result {
	// Export the PDF / PNG.
	Ok(document) => Ok(export_pdf(&document, self)?),
	Err(errors) => Err(format_diagnostics(self, &errors, &warnings).unwrap().into()),