feat(Widget): wrapper for widget.json

[Thumuss]

Please describe the changes this PR makes and why it should be merged:
Okay, I'm remaking a new pr because my last was really terrible + i've deleted the original (#5030)

My goal is to add Widget in discord.js
Widget is a part of Discord and it's not fully featured

Status and versioning classification:

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

Thanks to @ckohen @advaith1 @SpaceEEC @almostSouji @vladfrangu @NotSugden @kyranet @Mesteery for helping me in the last pull request

[Thumuss]

It's a mess in the logs but I've added in client an Promise.reject in Client#fetchWidget + clean up the if condition in the Widget class

[Thumuss]

I have add a new function, exactly the same as the Client#fetchWidget but in the Widget to update the widget
And I fixed the jsdoc like you said vladfrangu

[Thumuss]

I have done a test but that change anything I guess ? I've just link my local to my github project (3ead9e6)

Stale

[SpaceEEC]

Otherwise LGTM

Stale

[kyranet]

Just some formatting things for consistency, otherwise LGTM

[Thumuss]

Something I didn't see was the property fetchWidget in Guild (Guild#fetchWidget).
It's actually an other api request. I'm thinking about rename Guild#fetchWidget to Guild#fetchWidgetOption and re use the Client#fetchWidget for Guild#fetchWidget
It is a good idea or do I need to do something else ?

Stale

[Thumuss]

Before push it, do we need add a Guild#fetchWidget like I said upper ?

Stale