Regression in `decrypt` function return type

Before #29, the `decrypt` function would return a `Promise` with a generic `R`. It was changed to return `Promise` instead, which breaks the implementation in `snaps-monorepo`, because `unknown` is not assignable to the `Json` type, while the generic `R` was.

Locate the `decrypt` function and review the return-type change introduced by #29. Restore the generic `R` return contract described in the issue, then verify that the resulting type is assignable to the `Json` type used by snaps-monorepo.

Written by the indexing model from the issue text.