feat(*): use enums for consistency and speed

[iShibi]

Please describe the changes this PR makes and why it should be merged:

Okay, there are a lot of related changes here:

1. Use enums instead of arrays. This brings consistency and speed (indexOf is slow).
2. Some enums (that were arrays earlier) had same name for the enums themselves and the type of elements they hold.
3. WebhookTypes enum (ex-array) was missing from typings for Constants
4. Some methods that used arrays as enums earlier wouldn't let user to pass number type.
5. DefaultMessageNotifcations is now DefaultMessageNotificationLevels to keep the naming consistent with other enums in djs (discord uses 'level' in the name for enums in documentaion).
6. DefaultMessageNotifcations types were not same as what the api has documented ( eg. ALL instead of ALL_MESSAGES)
7. Although, the raw activity data's name property's value is Custom_Status, the type for it is documented as Custom. So, the activity type CUSTOM_STATUS is now CUSTOM.

Status and versioning classification:

• Code changes have been tested against the Discord API
• I know how to update typings and have done so
• This PR includes breaking changes

[kyranet]

Sorry, wrong button... ^

Stale

[iCrawl]

This needs a rebase.

[iShibi]

Rebased