verify that we fully support the segment template spec

[friday]

Currently we string replace $Number$ with the segment index, but there is also for example a zero-pad syntax $Number%0[width]d$ where you should zero-pad with the number specified as the width. I have fix for this coming up, but there seems to be more to it. For example if you look at dash.js issue tracker for PR 32 and 196, they have further details.

https://ottverse.com/structure-of-an-mpeg-dash-mpd/#Segment_Templates

#53 fixes the known one, but the dash.js pr 196 supports a lot more variants, and it would be good to also add tests.