Markdown extension: Preview display performance

[kieferrm]

For #6106:

A typical markdown editing flow is to rapidly switch back and forth between source and preview.

Comparing the built-in markdown preview with the extension, the extension has a perceivable lag when displaying the preview. We need to get back to the level of performance of the built-in editor.

[jrieken]

Problem is that the update-logic is only triggered on document save events, not on document change events. Also but smallish is an extra fs-read, instead of using the value form the TextDocument.

[kieferrm]

Summarizing offline discussions:
The issue here is the perceived lag when the webview is created. It takes on average 250ms.

[kieferrm]

No planned activity.