feat: add getGameLeaderboards() function

[stiefeljackal]

This PR adds a getGameLeaderboards function to the library to match what is available in the Game Leaderboards documentation. pnpm verify has passed, and the required tests are included in this PR.

PR for documentation update: RetroAchievements/api-docs#97

Resolves #115

[wescopeland]

Thanks!

[greptile-apps]

Confidence Score: 5/5

This looks safe to merge after normalizing numeric totals.

• String API totals can violate the exported response type.
• Request construction and exports match existing patterns.

src/leaderboard/getGameLeaderboards.ts

_{Reviews (1): Last reviewed commit: "Merge branch 'main' into feat/leaderboar..." | Re-trigger Greptile}

[greptile-apps]

Numeric totals bypass coercion

When the API returns string Count or Total values, callers receive strings despite the public numeric contract.

Suggested change

	return serializeProperties(rawResponse, {
	shouldCastToNumbers: ["ID", "Score"],
	shouldMapToBooleans: ["RankAsc"],
	});
	return serializeProperties(rawResponse, {
	shouldCastToNumbers: ["Count", "Total", "ID", "Score"],
	shouldMapToBooleans: ["RankAsc"],
	});