[discussion]: Put foundations on Octokit namespace

[oscard0m]

Description

To start defining and discussing the foundations of Octokit namespace.

Original messages:

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I gave this some more thought, and I think I'd actually prefer what express is doing by defining a global namespace, see https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/express-serve-static-core/index.d.ts#L17-L25

The reason for that is that it is possible that these type definitions will move to other packages in future, e.g. an @octokit/events package or @octokit/types. My understanding is that this would require changes to the declare "@octokit/webhooks" blocks, while the global namespace approach would always work

declare namespace Octokit {
    export interface Events {
        "my_custom_event": MyCustomEventPayload
    }
}

Originally posted by @gr2m in #250 (comment)

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In Typescript interfaces merging is allowed. I'll have to test how it'll pick up as the interfaces would be in different files. We might need to wrap the interface in a module.

_Originally posted by @ankeetmaini in #250 (comment)

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I want to give it more thought, a global Octokit namespace would affect all Octokit libraries. We can start experimenting now, I just don't want to rush ahead and build it, to avoid disruptive changes once we put it all together with the types from the REST API endpoints, etc

_Originally posted by @gr2m in #250 (comment)