merge multiple sprite files into a single sprite

[JinIgarashi]

This might be related to #29.

Combination of multiple style.json can be done manually, however currently there is no easy way to merge multiple source of sprite into a single sprite unless we generate sprite files from original SVG icons.

It would be great if charites have a feature to merge multiple sprite files although I am not very sure whether it is possible technically...

[keichan34]

Since maplibre introduced multiple stylesheets per style.json, I think this is no longer needed. I built a simple service to merge stylesheets, but we've deprecated it in favor of that feature. What do you think?