Support multiline string type, make LdtkAsset clonable, print the field name in unknown field error

[jrasanen]

Make LdtkAsset clonable

This improves the ergonomics when used with bevy_asset_loader

Improve error messages for unknown fields

Add the name of unsupported field into the error message

Multilines

Support Multilines -type, it's identical to the string, but it can contain Markdown formatting in the LDtk editor:

{ "__identifier": "label", "__value": "Strange crate", "__type": "String", "__tile": null, "defUid": 65, "realEditorValues": [{
    "id": "V_String",
    "params": ["Strange crate"]
}] },
{ "__identifier": "description", "__value": "The crate has some military logo on it.", "__type": "Multilines", "__tile": null, "defUid": 64, "realEditorValues": [{
    "id": "V_String",
    "params": ["The crate has some military logo on it."]
}] },

[Trouv]

Sorry for taking so long to get back to this. Could you cargo fmt it before I merge?

[jrasanen]

Hey! No worries, I've been busy with life, I'll try to remember to do it as soon as possible.

[Trouv]

Thanks! Merging