refactor: only cache commands from own user

[ThaTiemsz]

Please describe the changes this PR makes and why it should be merged:
The applicationCommandCreate/Update/Delete events are sent when guild commands owned by any bot in the server are created, updated, or deleted - and not just the commands owned by the client user. This meant that Discord.js was caching commands owned by other bots.

This PR:

• makes sure the command events only cache commands from the own bot (thanks @vladfrangu for the code suggestions)
• adds ApplicationCommand#applicationId
• modifies some of the ApplicationCommandManager methods so that they don't patch the cache in place (this fixes a bug where the applicationCommandUpdate event would emit the oldCommand with the new data), unless the targeted command is a global command. (The command events aren't fired for global commands, only guild commands)

Status and versioning classification:

Please move lines that apply to you out of the comment:

• 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)