chore(gatsby): Convert page-data-stats reducer to TypeScript

[carrotderek]

Description

This PR converts packages\gatsby\src\redux\reducers\page-data-stats.js to TypeScript and updates related files.

Related Issues

Related to #21995

[pieh]

Awesome, thank you!

And thanks for resolving conflicts that I saw yesterday (was planning to not bother you with that and do it myself but you beat me to it! :D)

[gatsbot]

Holy buckets, @carrotderek — we just merged your PR to Gatsby! 💪💜

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