Add default projection of "OSMTILE" to a <mapml-viewer> with no projection attribute

To stay consistent with the [spec](https://maps4html.org/MapML/spec/#the-mapml-viewer-element-0).

https://github.com/Maps4HTML/Web-Map-Custom-Element/blob/7b0858f29dc9f668d7ed104b7e894485ee966884/src/mapml-viewer.js#L76-L95