Rename `justification` settings to `just` settings

[teunbrand]

This PR aims to fix #6607.

I'll reiterate that this is just for consistency purposes. I know from previous revdepchecks that some packages save their themes to objects instead of regenerating them by calling theme(). Other packages have expect_silent() in their test, which is incompatible with lifecycle changes. Such packages are at risk of being negatively affected.

[teunbrand]

If the code is building a <theme> object, no. If they're reading out a <theme> object, it might. I could be persuaded that this is a risky move that is not worth the intended benefit of consistency.

[thomasp85]

Ah - I see now how you deal with the old spelling inside the code... I'm fine with putting this in the code base - if you want we can run a revdepcheck on it before merging?

[teunbrand]

Yeah I think a revdepcheck would be wise for this.