Add missing extension in import path

[jailln]

Add missing js extension in import path causing the following issue when using 3d-tiles-renderer 0.3.33 in an external project:

ERROR in ./node_modules/3d-tiles-renderer/src/three/gltf/classes/StructualMetadata.js 4:0-72
Module not found: Error: Can't resolve './PropertyAttributeAccessor' in '/home/jailln/Documents/dev/itowns/node_modules/3d-tiles-renderer/src/three/gltf/classes'
Did you mean 'PropertyAttributeAccessor.js'?
BREAKING CHANGE: The request './PropertyAttributeAccessor' failed to resolve only because it was resolved as fully specified
(probably because the origin is strict EcmaScript Module, e. g. a module with javascript mimetype, a '*.mjs' file, or a '*.js' file where the package.json contains '"type": "module"').
The extension in the request is mandatory for it to be fully specified.
Add the extension to the request.
[...]

PS: Thanks for this great lib 🙂

[gkjohnson]

Great, thank you!