feat(InteractionCreate): move to an Action handler

[monbrey]

Ignoring the much larger issues about Websocket and Action handlers still being separate, this moves the INTERACTION_CREATE logic over to an Action handler.

This allows Action#getChannel to be used for caching a partial DMChannel, preventing Interaction#channel from returning null with the CHANNEL partial enabled

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

[vladfrangu]

Meant to request changes, ^

[ImRodry]

The typings for Interaction#channel need to be updated to add null (seems like this was never added for some odd reason)

discord.js/typings/index.d.ts

Line 506 in 1816a93

public channel: TextChannel | DMChannel | NewsChannel;

This should be

 public channel: TextChannel | DMChannel | NewsChannel | PartialDMChannel | null; 

Stale