Implement Adjacency Connections for Counters

[Ryan089]

Summary

Like other connectables, counters are able to connect to form a coherent monolithic structure as described in the "Counter Adjacency Connections" section of the Connectables Design Document. Implement the adjacency connector script to enable this. See MultiAdjacencyConnector.cs and IAdjacencyConnector.cs as a start point.

Goal

Counters can be placed in the Tilemap editor, with the appropriate mesh being selected automatically to form the appropriate shape based on the adjacent tiles.

Dependencies

Assets required for this task will be added via #796.

[Notterox]

Hi, @Ryan089, assign me, please.

[Em3rgencyLT]

Depends on #865