[subyt.source] track and expose filesystem-location-info on the source (and if possible also the sink?) to the templates

[mpo-vliz]

We know where sources are loaded from.
And even a bunch of other metadata from that (coming from the filesystem and the pysubyt coding)

• from fs: lastmod, its fullpath, relative access path, mimetype, ...
• from subyt: type of source, passed config stuff, ..

We could expand this information via the template processing context
e.g. like {{ set['_'].meta.path }}

Note: some careful thought required on relative paths and making them into absolute (or relative?) uri references...

[mpo-vliz]

links also to #63