core: load user system config

[T1mVo]

Summary

See commit messages.

Prior work for #315

Checklist

• I have structured my commits according to the commit guidelines (if applicable).
• I have added tests for new features (if applicable).
• I have updated the documentation (if applicable).

[T1mVo]

@tingerrr I don't think there's much work to do here, but please have a look at whether SystemConfig should be attached to Project or rather somewhere else.

[tingerrr]

[...] I don't think there's much work to do here, but please have a look at whether SystemConfig should be attached to Project or rather somewhere else.

I think it's fine to add to Project since it is currently treated like a context object for commands that operate inside an active Typst project.

[T1mVo]

I've renamed config to project_config in most places. I can drop that commit, though, if this doesn't seem right.

The pipeline failed for some reason, but a rerun might already fix it.

[tingerrr]

The pipeline failed for some reason, but a rerun might already fix it.

Yes, that one job is fairly flaky.

Could you use just project: for the commit descriptions? I don't find conventional commits to be all that useful.

The rest looks good.

[T1mVo]

Done

[tingerrr]

Thank you!