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/tests/commands/drop.rs

@@ -36,7 +36,7 @@ fn more_columns_than_table_has() {
               [3,  white]
               [8, yellow]
               [4,  white]
-            ] | drop column 3 | columns | empty?
+            ] | drop column 3 | columns | is-empty
         "#)
     );