Add msecs-to-timestamp as a common function

[sachac]

That way, calling it from a WebVTT file uses the right format.

• subed/subed.el (subed--generic-function-suffixes): Add msecs-to-timestamp.
• tests/test-subed-srt.el ("Converting msecs to timestamp"): Add test.
• tests/test-subed-vtt.el ("VTT"): Add test.

[rndusr]

Thanks!