support copy/pasting assets between projects

[riknoll]

fixes microsoft/pxt-arcade#6656

this changes the asset fields' serialization so that they also include any referenced assets when saving state. saveState/loadState aren't called on fields during XML serialization, but they are called during copy/paste so this handles that scenario nicely without changing the save format of our projects.

when loading the state of a saved asset, there are a few scenarios to consider:

1. pasting the asset into a project that doesn't have an existing asset with that id
2. pasting the asset into a project that does have an asset with that id, where the value of the existing asset matches that of the serialized asset
3. pasting the asset into a project that does have an asset with that id where the value of the existing asset does not match that of the serialized asset

for scenario 1, we simply create a new asset in the destination project. for scenario 2, we don't do anything since the asset is already in project.

scenario 3 is the tricky one. first we check to see if there is another asset in the destination project with the same value. if so, paste that one instead. if there isn't, then we need to create a new asset with a different id to avoid the collision. luckily, we already have code to do this for tilemaps since we have supported copying/pasting from within the tilemap editor for a while now