[extractor] Support default implicit graph in JSON-LD (#1983)

[extractor] Support default implicit graph in JSON-LD (#1983)

Original PR: https://github.com/ytdl-org/youtube-dl/pull/30229

Per W3C JSON-LD v1.1 §4.9 (non-normative ref):

    When a JSON-LD document's top-level structure is a map that contains
    no other keys than @graph and optionally @context (properties that
    are not mapped to an IRI or a keyword are ignored), @graph is
    considered to express the otherwise implicit default graph.

Authored by: zmousm