Add missing support for addition and subtraction

[nilshg]

Inspired by the discussion in #28570.

Tests pass locally with this change.

There are no additional tests for missing support in this PR, it seems to me the methods are so simple that I'm not sure what would be tested here (although I have a feeling saying things like this is why I'm not a software developer...)

[pdeffebach]

You need <, >, <=, and >= as well, right?

[nilshg]

You need <, >, <=, and >= as well, right?

Those are covered in missings.jl in Base as per discussion on Slack, right?

[pdeffebach]

Yes, no need to add them.

[nalimilan]

Thanks, looks good to me.

As discussed at #28570, I think this is justified as all types that support + and - should propagate missing. This is different from adding new missing-propagating functions, which I agree is problematic as it's not clear where to stop. Here the limit is very clear: add methods to all functions that already propagate missing values to ensure consistency.

Fun fact: this missing method was also spotted by pandas developers, who have recently worked on improving their missing values handling and took Julia as a reference.

[nilshg]

Thanks, looks good to me.

Thanks for your help!