Improve with-env robustness (#12523)

# Description
Work for #7149

- **Error `with-env` given uneven count in list form**
- **Fix `with-env` `CantConvert` to record**
- **Error `with-env` when given protected env vars**
- **Deprecate list/table input of vars to `with-env`**
- **Remove examples for deprecated input**

# User-Facing Changes

## Deprecation of the following forms

```
> with-env [MYENV "my env value"] { $env.MYENV }
my env value

> with-env [X Y W Z] { $env.X }
Y

> with-env [[X W]; [Y Z]] { $env.W }
Z
```

## recommended standardized form

```
# Set by key-value record
> with-env {X: "Y", W: "Z"} { [$env.X $env.W] }
╭───┬───╮
│ 0 │ Y │
│ 1 │ Z │
╰───┴───╯
```

## (Side effect) Repeated definitions in an env shorthand are now
disallowed

```
> FOO=bar FOO=baz $env
Error: nu:🐚:column_defined_twice

  × Record field or table column used twice: FOO
   ╭─[entry #1:1:1]
 1 │ FOO=bar FOO=baz $env
   · ─┬─     ─┬─
   ·  │       ╰── field redefined here
   ·  ╰── field first defined here
   ╰────
```

crates/nu-command/tests/commands/let_.rs

@@ -63,7 +63,7 @@ fn let_pipeline_redirects_externals() {
 #[test]
 fn let_err_pipeline_redirects_externals() {
     let actual = nu!(
-        r#"let x = with-env [FOO "foo"] {nu --testbin echo_env_stderr FOO e>| str length}; $x"#
+        r#"let x = with-env { FOO: "foo" } {nu --testbin echo_env_stderr FOO e>| str length}; $x"#
     );
     assert_eq!(actual.out, "3");
 }
@@ -71,7 +71,7 @@ fn let_err_pipeline_redirects_externals() {
 #[test]
 fn let_outerr_pipeline_redirects_externals() {
     let actual = nu!(
-        r#"let x = with-env [FOO "foo"] {nu --testbin echo_env_stderr FOO o+e>| str length}; $x"#
+        r#"let x = with-env { FOO: "foo" } {nu --testbin echo_env_stderr FOO o+e>| str length}; $x"#
     );
     assert_eq!(actual.out, "3");
 }