Improve `pieceTracker` typing

Currently, `Match#pieceTracker` is an array—at some point I'd like to convert it to something like a `T extends Record` so that referring to a specific piece tracker is more readable.

Start by locating the Match#pieceTracker property and the GamePieceTracker type in the TypeScript library. Review how piece trackers are currently accessed, then define the expected typed representation and verify that specific tracker references are more readable without breaking existing usage.

Written by the indexing model from the issue text.