Related to #8368.
# Description
as planned in #8311, the `enter`, `shells`, `g`, `n` and `p` commands
have been re-implemented in pure-`nushell` in the standard library.
this PR removes the `rust` implementations of these commands.
- all the "shells" tests have been removed from
`crates/nu-commnand/tests/commands/` in
2cc6a82da6, except for the `exit` command
- `cd` does not use the `shells` feature in its source code anymore =>
that does not change its single-shell behaviour
- all the command implementations have been removed from
`crates/nu-command/src/shells/`, except for `exit.rs` => `mod.rs` has
been modified accordingly
- the `exit` command now does not compute any "shell" related things
- the `--now` option has been removed from `exit`, as it does not serve
any purpose without sub-shells
# User-Facing Changes
users may now not use `enter`, `shells`, `g`, `n` and `p`
now they would have to use the standard library to have access to
equivalent features, thanks to the `dirs.nu` module introduced by @bobhy
in #8368
# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- ⚫ `toolkit test`
- ⚫ `toolkit test stdlib`
# After Submitting
the website will have to be regenerated to reflect the removed commands
👍
