Update preview automatically as you save?

Now we have one option `Update preview automatically as you type` to let MacDown update preview during typing.

But sometimes this real time rendering will make typing not responding, so I have no choice to opt out this feature.

Can we introduce one alternative option like "Update preview automatically as you save" to improve the performance?

Start by locating the existing “Update preview automatically as you type” setting and the code that handles saving in MacDown. Confirm how preview updates are triggered, then ensure the new save-based option updates the preview after saving without rendering on every keystroke.

Written by the indexing model from the issue text.