feat(Guild): add fetchWidget() for getting widget data

[advaith1]

Please describe the changes this PR makes and why it should be merged:
Currently, you can fetch a guild's widget data with Client#fetchWidget(guild), but you can't do it from the guild object. Now, you can use Guild#fetchWidget() as a shortcut. The old fetchWidget and setWidget methods have been renamed to fetchWidgetSettings and setWidgetSettings to more accurately explain their functionality.

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)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[DTrombett]

Mmh, I see that there are methods on the Client object like fetchGuildPreview and fetchGuildTemplate that start with fetchGuild as they're related to guilds, so wouldn't be better to rename Client#fetchWidget() to Client#fetchGuildWidget() too?

[advaith1]

hm, maybe? cc @discordjs/the-big-3 (btw does this actually ping?)

[vladfrangu]

hm, maybe? cc @discordjs/the-big-3 (btw does this actually ping?)

1. Yes, yes it does ping
2. That sounds like a change I'd be ok with, waiting on the other two to reply tho 👍

[iCrawl]

2. Sounds good.

[SpaceEEC]

Sounds good to me too.