Added polyfills for `Map.getOrInsert()` and
`Map.getOrInsertComputed()` from the TC39 upsert proposal and refactor
the codebase to use them.
These methods replace the common "check if key exists, then set default"
pattern with a single atomic operation. This improves code readability
and eliminates redundant map lookups throughout the player.
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Co-authored-by: Álvaro Velad Galván <ladvan91@hotmail.com>