support '=' in key=value pairs

[kevinushey]

Bug description

Originally from rstudio/rstudio#13634.

I believe this comes from

quarto/packages/editor/src/api/pandoc_attr.ts

Lines 313 to 316 in 2349d21

	return lines.map(line => {
	const parts = line.trim().split('=');
	return [parts[0], (parts[1] || '').replace(/^"/, '').replace(/"$/, '')];
	});

, where the entire line is split on =. However, because the value itself may also contain an =, this can end up truncating the retrieved value.