Move symbols to json

[spatialillusions]

For easier reuse in other programming languages, all symbol definitions should be moved to JSON files using something similar to Template literals for colors and such. In the best of worlds this would make it possible to remove the symbol cache as well. This will not be done until milsymbol 2.0 has been released.

[dehmer]

@Måns I'm currently in the process of trying exactly this for icons and icon parts. That's quite challenging at least for icon parts since they depend on a bunch of runtime/configurations options (standard, sidc type, style options, e.g. civilian colors. I could report back experiences I made on the way. That is, if it's still a topic of interest.