[XFileShare] Update for 2024 with new InfoExtractor features

[dirkf]

Boilerplate: improvement/own code

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• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

This PR updates the XFileShare extractor module in three steps:

1. adds a _search_json() method to InfoExtractor matching the yt-dlp method; it uses the error diagnostic to truncate extra text from the JSON string, which might be confused by a Python json library implementation that uses a different diagnostic template;

2. updates the common JWPlayer extraction to use traverse_obj() and _search_json(), to support extracting playlists in a .load({**video1},{**video2}, ...) fragment, and to support a transform_source parameter in _extract_jwplayer_data();

3. updates the XFileShareIE extractor to make use of these features.

As a result, filemoon.sx can be supported as one of the known sites and its separate extractor is removed: resolves #32716.

Supersedes, closes #32452.