chore(gatsby): migrate themes reducer to TypeScript

[taejs]

Description

migrate themes reducer to TypeScript

Related Issues

Related to #21995

[taejs]

I'd like to change type of themes property any to custom type.

gatsby/packages/gatsby/src/redux/types.ts

Line 192 in e643d6b

themes: any // TODO

Then I have to debug bootstrap/index.js,
How can I debug it? I've tried, but It can't be tested

[ascorbic]

@taejs Adding types shouldn't require any debugging of js files, as long as you aren't changing any runtime behaviour or exports. I'd do this in another PR as part of a migration of bootstrap/load-themes rather than as part of this.

[gatsbot]

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