REFACTOR: remove the shell commands (#8415)

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
👍

crates/nu-std/src/lib.rs

@@ -88,6 +88,7 @@ pub fn load_standard_library(
         // Conventionally, for a command implemented as `std foo`, the name added
         // is either `std foo` or bare `foo`, not some arbitrary rename.
 
+        #[rustfmt::skip]
         let prelude = vec![
             ("std help", "help"),
             ("std help commands", "help commands"),
@@ -95,6 +96,13 @@ pub fn load_standard_library(
             ("std help modules", "help modules"),
             ("std help externs", "help externs"),
             ("std help operators", "help operators"),
+
+            ("enter", "enter"),
+            ("shells", "shells"),
+            ("g", "g"),
+            ("n", "n"),
+            ("p", "p"),
+            ("dexit", "dexit"),
         ];
 
         let mut working_set = StateWorkingSet::new(engine_state);