Import .geojson-files with custom Asset importer

[ElmarJ]

Currently, we import the geojson-files as text assets using the .json extension.

• One obvious disadvantage is that this means that UI only accepts .geojson-files with a .json extension. The .geojson extension is more common and should be supported as well.
• On a larger scale, it would be an important step to allow two-way link between the geojson and the unity-app, enabling scenario's where in-editor changes and even runtime-changes to the maps, could be propagated back into the original geojson-files (e.g. on a git server).

Resources / interesting reads:

• https://blog.innogames.com/building-a-custom-asset-pipeline-for-a-unity-project/

[ElmarJ]

This would mean not using TextAsset. Moving away from TextAsset would actually be a good idea, as it would also allow us te keep the .geojson-files out of the runtime assets - they are not used at runtime anyway.

[ElmarJ]

See https://docs.unity3d.com/Manual/ScriptedImporters.html for info on writing custom importers for specific file extensions. This would also make it easier to:

1. Put the git submodule with geojson-files directly in the assets folder.
2. Store metadata (such as original git-filelocation of the geojson file)