Recognise .forced in subtitle filenames

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Expected behavior of the wanted feature

This is similar to issue #14805 (fixed by #16020), but for forced external subtitles. It would allow users of external subtitles to benefit from your work on subtitle auto-selection when using the subs-with-matching-audio=forced option. As with the other issue, it'd also fix sub-auto and language detection when a filename ends with .forced.<ext>.

The filename is typically of the form <title>.<lang>.forced.<ext> (as used by Plex), but I've also seen it in the form <title>.forced.<lang>.<ext>.

Please also add forced as an flag to the sub-add command.

Alternative behavior of the wanted feature

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Log File

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Sample Files

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