feat: Perform the computation in a separate buffer before showing it

[Konubinix]

The previous implementation first killed the preview buffer and then run plantuml. If plantuml takes a few seconds, it is annoying to have the buffer vanishing and appearing again a few seconds later.

It seems smoother to perform the computation while still showing the obsolete buffer and replace the buffer when the result is ready.

[Konubinix]

BUMP