chore: _uid is optional on time objects
feat(TiledTMX): support serialization of tiled map file

- add write() method that returns an XML string representing the file.
- add tests to compare an originally loaded map to a clone of it that has been written out as a string and parsed and re-loaded by the resource loader.   In this way it is possible to test what is read vs written, without worrying about things like whitespace and empty tag formatting, etc.  Just check the actual data.
- collapse extra .. and . in relative paths, to keep the exported name for maps and tilesets as simple as possible.
chore: bring over a few error constants from pow-edit
chore: add artifacts for release
chore(deploy): release v0.1.5