Add property type to SlashCommandBuilder

[immjs]

Please describe the changes this PR makes and why it should be merged:
Might help if people want to categorize their commands, instead of checking if the builder is a SlashCommandBuilder, they can simply check the type property of builder, very much like how ContextMenuCommandBuilder does it

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)

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[Qjuh]

This is not consistent builders design. You use the builder while you want to add to it/change it. When you‘re done you can .toJSON() it and will have an object having that type set already.

[immjs]

I had spent a few minutes looking at the toJSON and there is nothing adding type automagically. Simply, it is not required by the API.

If you want to talk about design, we can also talk about how ContextMenuCommandBuilder does have a type property yet SlashCommandBuilder doesn't, even though they end up in the same API call.

[monbrey]

Sorry, I have to disagree. ContextMenuCommandBuilder has a type property because there are two types that have no other differences between them, and it's required by the API to define an application command as those types.

For the use-case you described developers can simply add a type property, category property, or literally any other property they like alongside the builder in the exports.

[immjs]

Sorry, I have to disagree. ContextMenuCommandBuilder has a type property because there are two types that have no other differences between them, and it's required by the API to define an application command as those types.

For the use-case you described developers can simply add a type property, category property, or literally any other property they like alongside the builder in the exports.

Well, what are the downsides?

[jaw0r3k]

Well, what are the downsides?

There is just no need to add it

[kyranet]

The ContextMenuCommandBuilder class has a type property because it uses the old builders design, which we will phase out on builders v2 based on the RFC.

You may use instanceof instead, or call toJSON() to read the type.