fix(Guild): cache fetched widget data

[NotSugden]

Please describe the changes this PR makes and why it should be merged:
This PR modifies the Guild#fetchWidget function to cache the data received as it is not sent in the initial GUILD_CREATE packets (API Docs ref)

this PR also changes setEmbed and fetchEmbed to instead return the respective widget function,
this was to avoid duplicated code - both methods do technically call a different endpoint but they yield the same result
Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.