fs feature: support subs/ subfolder

[chapmanjacobd]

Is your feature request related to a problem? Please describe.
Some of my videos have a subs/ subfolder with external subtitles

Describe the solution you'd like
A few new functions to search for those common patterns and make the subtitle files known (part of the fs_extract:external_subtitles array) for extraction into the tags column

Additional context
Very low priority for me because fewer than 1 percent of folders that I have are structured like that

[chapmanjacobd]

I don't think this is possible to implement in a fast way