fix: Improve GNU sed fallback

[hyperupcall]

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Code inspired by this comment, this PR fixes the sed command on BSD/Busybox, etc. platforms in which GNU sed is not used.

Unlike what some comments in the linked issue suggests, the approach of exporting the function so todotxt add-ons can automatically use the fix was not done. This is because that would be a breaking change and there is no way we can guarantee that the assumptions of this sed() function hold true for all plugin code.

How to test:

make test

Closes #168

[inkarkat]

On Ubuntu (and I think Cygwin as well), there's no gsed command, only (GNU) sed. The sed() wrapper would use the in-place emulation there, which I think would be a bad side effect. Auto-detecting GNU sed is difficult and has a performance impact as well - I don't want to go down that route. In #447 (created because you don't allow pushes to your fork), I've tried to address this via new config variables. What do you think about that?

[hyperupcall]

What do you think about that?

I think it's better compared to the original code, but like you mention, it still uses the sed() wrapper by default (TODOTXT_SED_COMMAND is unset by default), which is probably suboptimal. But I have an idea to address that!- I'll comment on the new PR

created because you don't allow pushes to your fork

I would if I could, but unfortunately, GitHub does not allow pushes to my fork because the repository is owned by an organization instead of a user 😭