feat(StageChannel): add createStageInstance method & use better naming convention

[iShibi]

Please describe the changes this PR makes and why it should be merged:

This PR adds a new createStageInstance method on StageChannel that lets the user create a stage instance associated to it. It also refactors the options typedef to keep it consistent with rest of the lib:

1. CreateStageInstanceOptions is now StageInstanceCreateOptions
2. The channel field is a seperate param now instead of being inside the options object

⚠️ As pointed out by @SpaceEEC, the term "instance" makes it sound like it is an instance of a Stage Channel which it isn't. This PR removes this ambiguity by using the complete term i.e. "Stage Instance" in all places.

Status and versioning classification:

• Code changes have been tested against the Discord API
• I know how to update typings and have done so
• This PR changes the library's interface
• This PR includes breaking changes

[kyranet]

I don't agree with the breaking change as the object still has a required property (topic).

If it's because StageChannel#createStageInstance (probably should be createInstance), you can do create({ ...options, channel: this.id }).

[iShibi]

It will be the same situation like MessageOptions again, where messageReference was getting set to this.id and hence ReplyMessageOptions was created. Unlike there, where we couldn't make messageReference a seperate param, it is much more intuitive to have channel as a first param to stageInstances#create here.

createInstance sounds good tho, will rename it.

[iShibi]

Refactored some stuff to reflect the new naming convention for things related to stage instances. Added more information covering this change in the description above and changed the title.

"instance of stage channel" What was I thinking pepehands

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