api: add missing slash command endpoints

[mavolin]

Closes #204.

[diamondburned]

Seems like this PR breaks some examples. Can you fix them?

+ go build ./_example/advanced_bot ./_example/buttons ./_example/commands ./_example/sharded ./_example/simple ./_example/undeleter
# github.com/diamondburned/arikawa/v3/_example/buttons
_example/buttons/main.go:38:6: cannot use option.NewNullableString("This is a message with a button!") (type *option.NullableStringData) as type string in field value
_example/buttons/main.go:39:6: cannot use &[]discord.Component{...} (type *[]discord.Component) as type []discord.Component in field value
_example/buttons/main.go:88:5: cannot use option.NewNullableString("Custom ID: " + e.Data.CustomID) (type *option.NullableStringData) as type string in field value
# github.com/diamondburned/arikawa/v3/_example/commands
_example/commands/main.go:36:5: cannot use option.NewNullableString("Pong!") (type *option.NullableStringData) as type string in field value

(see CI logs)