fix: Ensure #| is added only at the beginning of a new line

[mcanouil]

Adjust the insertion logic to guarantee that #| is added at the start of a new line, addressing issues with code formatting on return. Fixes #426

Proposed behaviour:

Screen.Recording.2025-02-03.at.14.13.16.mov

[mcanouil]

@cscheid @juliasilge gentle ping for this as it’s really a daily pain to have to fix the bad insertion.

[juliasilge]

I took a look at this, and we don't see the bad behavior from #426 in Positron because of onEnterRules for R and Python files, which get piped through and applied via the virtual documents that Quarto creates. So this change is not really applicable for Positron. I do think we'll eventually need to resolve the edit builder here with onEnterRules that other extensions can provide.

I do see the bad behavior in VS Code, and this PR fixes the problem. ✅

[mcanouil]

I do think we'll eventually need to resolve the edit builder here with onEnterRules that other extensions can provide.

I do agree 100% with that but this means quite a change in the VSCode extension codebase which I was not willing to undertake at this point, especially without any tests.
The PR is really here to simply fix the current implementation not to improve/refactor it.