zellij-tile: Move data to zellij-utils

The rationale behind this is that all components of zellij access the
data structures defined in this module, as they define some of the most
basic types in the application. However, so far zellij-tile is treated
like a separate crate from the rest of the program in that it is the
only one that doesn't have access to `zellij-utils`, which contains a
lot of other data structures used throughout zellij.

This poses issues as discussed in
zellij-org#1242 and is one of the reasons
why the keybindings in the status bar default plugin can't be updated
dynamically. It is also the main reason for why the keybindings are
currently passed to the plugin as strings: The plugins only have access
to `zellij-tile`, but since this is a dependency of `zellij-utils`, it
can't import `zellij-utils` to access the keybindings.
Other weird side-effect are that in some places `server` and `client`
have to access the `zellij-tile` contents "through" `zellij-utils`, as
in `use zellij_utils::zellij_tile::prelude::*`.

By moving these central data structures to one common shared crate
(`zellij-utils`), `zellij-tile` will be enabled to import `zellij-utils`
like `screen` and `client` already do. This will, next to other things,
allow dropping a lot of `std::fmt::Fmt` impls needed to convert core
data structures into strings and as a consequence, a lot of string
parsing in the first place.