Expose what a link resolved to, and the safe reference read behind it

[romain-pm]

Part of #749. The link tier knows things it refuses to tell the caller.

• Return the resolved target on LinkProps. jahia.com needs it for a label fallback and could not get it, so it hand-wrote the reference read.
• Export the safe reference reader. resolveContentLink has a private readReference — a try/catch around getProperty().getValue().getNode() — and it is the thing that stops a dangling reference from throwing. It is exactly what every module needs and it is not exported. jahia.com re-wrote it in 8 lines in NavBar.tsx.
• Consider making it a first-class library export next to getNodeProps: reading a reference safely is not a link concern, it is a JCR concern. Any view touching a weakreference has the same problem, because an unresolvable reference reaches JavaScript as a plain falsy value with no way to tell "unset" from "unpublished" from "forbidden".