refactor(buffer): simplify set_stringn logic

[EdJoPaTo]

Simplifies the logic. Instead of trying to figure out the max_offset in a seemingly complicated way keep the remaining_width and count down as long as there are characters left. This is way easier to understand for me what is going on.

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Part of the improvements I did with #1049. As this is without _opt methods this can still panic (same as current code on wrong input).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.1%. Comparing base (366c2a0) to head (46ef759).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1083 +/- ## ======================================= + Coverage 89.4% 94.1% +4.7% ======================================= Files 61 61 Lines 15440 14619 -821 ======================================= - Hits 13808 13764 -44 + Misses 1632 855 -777 ```

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