feat: support JSON content model pages

[lcw861018]

Fixes pulling pages whose content model is \json\ (e.g. \MediaWiki:*.json).

Problem

When pulling a page with content model \json, \getPageCode\ mapped it to the \json\ language, but \commentList\ had no \json\ entry, so \getInfoHead\ threw \Unsupported content model: json\ and the page could not be opened.

Change

• In \modelNameToLanguage, map the \json\ content model to the \jsonc\ language (same highlighting as JSON, and the [PAGE_INFO]\ comment header is valid there), matching the existing handling of \low-board.
• Added a unit test covering round-tripping a JSON page header through \getContentInfo.

Verification

• 
  pm run compile\ passes (only a pre-existing webpack warning).
• Unit tests pass (4 passing, including the new JSON case).