feat(Interaction): add isApplicationCommand check

[manuelvleeuwen]

Please describe the changes this PR makes and why it should be merged:
Introduces Interaction#isApplicationCommand, which indicates whether or not the interaction is a BaseCommandInteraction (as of now the only subclasses are CommandInteraction or ContextMenuInteraction, but there might come more in the future).

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)

[iCrawl]

This is a semver-major and seems too big of a change, considering we don't even know when we'll release the next major and what has changed on Discords side to maybe justify this.

[manuelvleeuwen]

Yeah, I agree that renaming might be too big of a change, but still a check like Interaction#isApplicationCommand would be useful, for example: I defer all application command interactions by default (I'm making API calls that delay for quite long) and I can run them without checking whether they are slash commands or context menus.

interaction.isCommand() || interaction.isContextMenu()

also works but imo a single check for this is cleaner.

[ImRodry]

You can add the isApplicationCommand method just fine without needing to rename the class