Handle font sizes that are set as strings

[etimberg]

Resolves #6373

[benmccann]

Hmm, that issue is saying that there's an inconsistency between the legend and title fontSize where legend accepts a string and title doesn't. From a glance at the code, I don't see where legend accepts a string, so we might want to make sure that it still does and we're not introducing an inconsistency here. But if it does, I'd probably go the other direction and make legend fontSize only accept number. I'm not sure I want to find every option that takes a number and make sure they all work with string

[etimberg]

I don't think either accept a string before this PR. They may have previously done so, but _parseFont couldn't handle it so if the title supported a string it was broken before. Since all font parsing is done with _parseFont, I think any font size could now be a string

[etimberg]

In terms of accepting or rejecting invalid parameters, I'm not sure what options we have. We'd need some kind of pass to reject these and spit out useful error messages.