Rework multi-line output handling in the worksheet

[smarter]

Even after #5552 there were still situations were the worksheet output
was not displayed properly due to all the complications needed to handle
multi-line output.

Getting this right seems hopeless until vscode gets some better
API (microsoft/vscode#63600), so instead in
this commit we avoid the problem by only displaying the first line of
multi-line output. The complete output is still available by hovering
over the decoration which should be good enough.

[Duhemm]

LGTM

Have you considered showing (+n lines) or ⏎ or ↩️ instead of <truncated output>?

I think it may be clearer that more will be shown on hover if it says (+n lines).

[smarter]

I've settled for <n lines hidden>, it'd be even better if it was displayed in a different color but that would require using multiple decorations and I didn't want to add more complexity for now. Also when you're on the last line of the file and press Enter, if the line is decorated, then the decoration will be moved to the new last line of the file. I haven't found a way to avoid that and am wondering if it should be considered a vscode bug.