TailChainSwapMove should be able to select all possible tail chain swaps

TailChainSwapMoveSelectorConfig needs to have 2 valueSelectorConfigs instead of a entity and a value selector, so it can select a move that swaps 2 arcs in the same chain (so same anchor) for which the leftEntity is null (think of it being the anchor). Now that's only possible with the rightEntity being null, but that means that it's always the non-anchor side that's being reversed, never the anchor side.

Pitfalls:

• nearby selection
• pinning