Deprecate --numbered from for (#13112)

# Description

#7777 removed the `--numbered` flag from `each`, `par-each`, `reduce`,
and `each while`. It was suggested at the time that it should be removed
from `for` as well, but for several reasons it wasn't.

This PR deprecates `--numbered` in anticipation of removing it in 0.96.

Note: Please review carefully, as this is my first "real" Rust/Nushell
code. I was hoping that some prior commit would be useful as a template,
but since this was an argument on a parser keyword, I didn't find too
much useful. So I had to actually find the relevant helpers in the code
and `nu_protocol` doc and learn how to use them - oh darn ;-) But please
make sure I did it correctly.

# User-Facing Changes

* Use of `--numbered` will result in a deprecation warning.
* Changed help example to demonstrate the new syntax.
* Help shows deprecation notice on the flag

crates/nu-protocol/src/errors/parse_warning.rs

@@ -10,7 +10,7 @@ pub enum ParseWarning {
     DeprecatedWarning {
         old_command: String,
         new_suggestion: String,
-        #[label("`{old_command}` is deprecated and will be removed in 0.94. Please {new_suggestion} instead")]
+        #[label("`{old_command}` is deprecated and will be removed in a future release. Please {new_suggestion} instead")]
         span: Span,
         url: String,
     },