Implement dynamic BSN (.bsn asset format)

[pcwalton]

This is a work-in-progress, but I'm submitting it to get early feedback.

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[pcwalton]

It was pointed out that we could try to share the Bsn and BsnEntry structures. The problem with that is that those structures are singly linked and we can't refer to individual pieces within them easily. That will cause problems because we want editors to be able to maintain bidirectional links. Perhaps we could make the Bsn tree use a World? It would mean that bevy_scene2_macros would have to depend on bevy_ecs so that it can make a World, but that should be doable?

@cart Would appreciate your feedback here.

[pcwalton]

cart mentioned on Discord that he prefers the design of this PR, where we have our own AST, parser, and lexer, because (1) it makes the differences, which are significant, between macro-BSN and asset-BSN more manifest and (2) because having a custom parser and lexer is less code to ship at runtime.