fix: scene exporter should export layers and tags while maintaining the tree structure

[rot1024]

fix reearth/reearth-visualizer#212

[codecov]

Codecov Report

Merging #104 (eaa9635) into main (b398db3) will increase coverage by 0.15%.
The diff coverage is 46.00%.

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   33.90%   34.05%   +0.15%     
==========================================
  Files         297      299       +2     
  Lines       26740    27040     +300     
==========================================
+ Hits         9065     9208     +143     
- Misses      16851    17000     +149     
- Partials      824      832       +8     

Impacted Files	Coverage Δ
internal/usecase/interactor/project.go	0.00% <0.00%> (ø)
internal/usecase/interactor/tag.go	0.00% <0.00%> (ø)
pkg/layer/id.go	0.00% <ø> (ø)
pkg/layer/merging/merged.go	0.00% <0.00%> (ø)
pkg/layer/merging/merger.go	63.52% <0.00%> (ø)
pkg/layer/merging/sealed.go	0.00% <0.00%> (ø)
pkg/layer/merging/sealer.go	0.00% <0.00%> (ø)
pkg/tag/item.go	24.00% <0.00%> (-22.16%)	⬇️
pkg/tag/loader.go	0.00% <0.00%> (ø)
pkg/tag/map.go	0.00% <0.00%> (ø)
... and 18 more