refactor: Switch `Shortcut::steam_id` from a field to a method #161

Job	Run time
wasm	30s
validation (ubuntu-latest, stable)	23s
validation (ubuntu-latest, beta)	38s
validation (macos-latest, stable)	20s
validation (macos-latest, beta)	44s
validation (windows-latest, stable)	2m 13s
validation (windows-latest, beta)	1m 28s
	6m 16s

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