Improve/callout title

[cscheid]

Rename quarto's Callout "caption" references. The term is confusing and used to mean different things.

This PR uses "title" for the callout "caption" references in Lua code and [S]CSS, and "localization" for the term used in our localization YAML files.

Specifically:

1. I've seen user code in the wild where users think caption means caption in the crossref sense of the term. That's not what callout "captions" are: they're really titles.
2. Even within the callout feature, we "caption" to mean different things. In the Lua and [S]CSS code, "caption" means the title of the callout. In the YAML files, "caption" means the name used for the different localizations. We now use title everywhere.

This PR breaks backwards compatibility, but I do feel that we need to make this change now rather than later, and more specifically, we should be precise about the terms we use in quarto. This will become more acute in the crossref reimplementation, at which point "caption" really should mean only "a textual description of a crossreferenceable artifact in the document".