refactor(model)!: clean up `AllowedMentions`

[vilgotf]

• flattens the allowed_mentions module into message
• removes AllowedMentionsBuilder (can be brought back into twilight-util if requested)
• renames ParseTypes to MentionType
• rewrites the AllowedMentions documentation

Follow-up PR to discussion in #1792

[vilgotf]

Note that these types can be moved to the http module, but I like their current placement.

[itohatweb]

Is there a reason of why the builder has been removed and has not been moved to util?

[vilgotf]

Is there a reason of why the builder has been removed and has not been moved to util?

I don't think there are any users of it that would not be happy with using ..Default::default(). I'm happy to be proven wrong

[itohatweb]

Imho it's just a bit easier to use the builder, but ig using default works too.

[7596ff]

I need to think more on what to do with the builder before I can approve this, but I've got my eye on it

[spring4175]

@7596ff just a ping to see if we can get this moving