Add TopDef property for specs

[janmasrovira]

This pr implements a request for the Specs team by @jonaprieto.

juvix dev highlight --format json now also outputs an additional field topDef that contains a list of pairs containing a file interval (with the usual encoding) and an identifier.
e.g

[["/home/jan/projects/juvix/juvix-stdlib/Stdlib/Extra/Gcd.juvix",10,1,4,10,4],"gcd"]

[jonaprieto]

In the specs, we are writing textual documents that require referencing Juvix terms. However, there is no way to easily link these terms, as the anchors generated by compiler are numbers.
After this PR, I plan to implement another case for the WikiLinks preprocessor for the mkdocs juvix plugin, so it can replace automatically these references. The candidate syntax may just prefix the term identifier with juvix: [[juvix:MyTerm]].

[jonaprieto]

LGTM. Although, you may want to add a smoke test of a minimal example and check that some expected term identifier appears in the JSON file.