Don't clobber other schema fields when attaching references (#15336)

This PR fixes an issue where we would clobber the other JSON Schema
fields for any field that we attached a reference to.

This resulted in these fields (e.g., `buffer_font_family`,
`ui_font_family`) losing things like their descriptions.

The approach has been adjusted that references are now added in an
additive fashion, rather than overriding the entire schema object.

Release Notes:

- Fixed an issue where font-related settings in `settings.json` were
missing their descriptions.