add DocumentLink support

[srivastava-diya]

Description

Adds textDocument/documentLink support: every $ref string in an open JSON/JSON Schema document becomes a clickable link that jumps to the AST node it points to.

• Resolves plain JSON Pointers (#/definitions/foo, #), $anchor references (#fooAnchor, including the legacy "$id": "#name" form), and embedded schemas identified by $id/id (with an optional JSON Pointer or anchor fragment).

• Resolves $refs that point to a different file, not just the current document. Already-open files are read from their live, and unsaved editor files that aren't open are read from disk and parsed fresh.

• Follows chained $refs , if a resolved target is itself {"$ref": "..."}, resolution keeps hopping, including across multiple files, until it lands on a real node. A visited locations guard makes circular $ref chains resolve to "no link" instead of hanging.

• JsonDocument.findNodeAtPointer gained an optional from parameter defaults to the document root so a pointer can be resolved starting at an arbitrary node , backward compatible, existing call sites unaffected.

[srivastava-diya]

I thought of using @hyperjump/browser's get() as the primary resolution mechanism, I used this same approach for my JRef Qualification task, but it turns out not to be effective here. SchemaStore already owns the single global application/json media-type, so get() gives back schema-compiler internals, not the plain JSON content of the file. I confirmed this by using step() on a .json file which returned {}

Registering a second application/json plugin isn't an option either.

Given all that, i added manual resolution.