discord: Added SelectedValue fields for Channel, Mentionable, User and Role SelectComponents

[CosmicPredator]

This PR adds the following fields to ChannelSelectComponent, MentionableSelectComponent and RoleSelectComponent,

• SelectedRoles for RoleSelectComponent.
• SelectedChannels for ChannelSelectComponent.
• SelectedMentions for MentionableSelectComponent.
• SelectedUsers for UserSelectComponent.

All the fields will take in the slice of ID's of their kind and populates the SelectComponent beforehand.

Note

SelectedMentions will take in the slice of interface{} and later type-asserted to respective UserID / RoleID.
If wanted to change this implementation, please lmk.

[diamondburned]

If we're using a hidden field, then it's best to just have separate hidden DefaultUsers and DefaultRoles fields, then combine them in MarshalJSON.

[MCausc78]

This breaks proper order. There might be {(user, 2), (role, 3), (user, 1)} sequence.

[CosmicPredator]

I agree with @MCausc78. What's wrong with using plain interfaces? It's already documented above the field for the users.

I can add an example snippet in the comments, If it's not clear enough

[diamondburned]

What's wrong with using plain interfaces? It's already documented above the field for the users.

It's not type-safe.

The only way I would agree to moving forward with this solution is to make custom SelectedUserID and SelectedRoleID types and a SelectableID union type to combine the two. Arikawa tries its best to not have any ambiguous/error-prone any types in its code, so this shouldn't be an exception.

[CosmicPredator]

@diamondburned How about this implementation?

type DefaultMention struct {
	userId *UserID `json:"-"`
	roleId *RoleID `json:"-"`
}

// Creates new DefaultMention type with only UserID
func UserMention (userId UserID) DefaultMention {
	return DefaultMention{userId: &userId}
}

// Creates new DefaultMention type with only RoleID
func RoleMention(roleId RoleID) DefaultMention {
	return DefaultMention{roleId: &roleId}
}

and when implementing,

DefaultMentions: []discord.DefaultMention{ discord.UserMention(793688107077468171), discord.RoleMention(1299037008332849212) },