Rename all?, any? and empty? (#6464)

Rename `all?`, `any?` and `empty?` to `all`, `any` and `is-empty` for sake of simplicity and consistency.

- More understandable for newcomers, that these commands are no special to others.
- `?` syntax did not really aprove readability. For me it made it worse.
- We can reserve `?` syntax for any other nushell feature.

crates/nu-command/src/env/env_command.rs

@@ -75,7 +75,7 @@ impl Command for Env {
             },
             Example {
                 description: "Check whether the env variable `MY_ENV_ABC` exists",
-                example: r#"env | any? name == MY_ENV_ABC"#,
+                example: r#"env | any name == MY_ENV_ABC"#,
                 result: Some(Value::test_bool(false)),
             },
             Example {