This patch implements the additional select menu types `user`, `role`,
`mentionable`, and `channel`. Moreover, it moves the type-specific data
for the existing text select menu implementation into a separate enum
variant.

The new types are implemented by "semi-flattening" the `SelectMenu`
struct: fields common to all select menu types are implemented in the
struct itself, and type-specific fields like the available options or
channel types are moved into a `SelectMenuData` enum. This enum is also
used to select a select menu's type.

This approach de-duplicates the common fields while preventing users
from accessing fields irrelevant to the current select menu type.

Finally, this commit updates the documentation and existing tests to
reflect these changes. The tests, however, might require additional
attention in case new behaviour introduced by this commit should be
tested as well.

This patch makes `ComponentVisitor::visit_map` use `unreachable!`
instead of `panic!` in case a select menu type was not implemented while
being listed in a previous match statement.

Moreover, this commit sneaks in a minor formatting fix inside
`Components` doc-comment example.

This commit implements the `resolved` field for
`MessageComponentInteraction`. This field holds resolved users, roles,
channels, or attachments for select menu interactions.

Unfortunately, the new field makes it impossible to implement `Eq` and
`Hash` for `MessageComponentInteractionData`.

This commit fixes the tests testing the (de-)serialization of
`InteractionDataResolved`. Previously, the tests were failing, as the
type was renamed from `CommandInteractionDataResolved`.

Moreover, this commit fixes the message component interaction tests by
adding the missing `resolved` field to the expected fields in
`message_component_interaction_data`.

This commit unboxes `SelectMenuData`'s variants. They were boxed in a
previous patch to prevent future fields from unnecessarily increasing
the enum's memory footprint, but considering that, with the current
structures, new fields are a breaking change already, it's OK to leave
the variants unboxed for now and re-box as necessary.

For the current code, this saves a heap allocation while only marginally
increasing the enum's memory footprint.

This change is only breaking in the context of PR twilight-rs#2219. When added to
the commits in this PR, it doesn't add any breaking changes to the PR's
list of breaking changes.

This commit removes the SelectMenuData type and embeds all
variant-specific fields in the base struct. Moreover, it adds
documentation indicating which fields are required variant-specific
fields.

This commit also updates the component validation module in
`twilight-validate` by introducing a new error type for text select
menus that don't have the necessary `options` field.

This commit streamlines the language in the `SelectMenu` and
`SelectMenuType` documentation. In particular, it removes duplicated
sections outlining required and optional fields. Moreover, it makes
language describing which menu types use a specific option more in
line with the rest of the library.