refactor(types): prefer type alias

[dai-shi]

I know this is mostly about preference, but I feel a little bad about mixing two styles for maintainability.
There are some differences between interface and type alias, like overloading, index signature and maybe others.
However, in the current code, mostly type alias is used and only some cases use interface. Hopefully making it consistent makes sense. (In this PR, to type alias.)
If this causes a real problem rather than styling preference, we can revisit. (I can think of one case, module augmentation, but wonder if anyone does it, and if it's the case, we should prefer "export function" too.)

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