Deprecate --flag: bool in custom command (#11365)

# Description
While #11057 is merged, it's hard to tell the difference between
`--flag: bool` and `--flag`, and it makes user hard to read custom
commands' signature, and hard to use them correctly.

After discussion, I think we can deprecate `--flag: bool` usage, and
encourage using `--flag` instead.

# User-Facing Changes
The following code will raise warning message, but don't stop from
running.
```nushell
❯ def florb [--dry-run: bool, --another-flag] { "aaa" };  florb
Error:   × Deprecated: --flag: bool
   ╭─[entry #7:1:1]
 1 │ def florb [--dry-run: bool, --another-flag] { "aaa" };  florb
   ·                       ──┬─
   ·                         ╰── `--flag: bool` is deprecated. Please use `--flag` instead, more info: https://www.nushell.sh/book/custom_commands.html
   ╰────

aaa
```

cc @kubouch 

# Tests + Formatting
Done

# After Submitting
- [ ] Add more information under
https://www.nushell.sh/book/custom_commands.html to indicate `--dry-run:
bool` is not allowed,
- [ ] remove `: bool` from custom commands between 0.89 and 0.90

---------

Co-authored-by: Antoine Stevan <44101798+amtoine@users.noreply.github.com>

crates/nu-parser/src/parser.rs

@@ -18,8 +18,8 @@ use nu_protocol::{
     },
     engine::StateWorkingSet,
     eval_const::eval_constant,
-    span, BlockId, DidYouMean, Flag, ParseError, PositionalArg, Signature, Span, Spanned,
-    SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
+    span, BlockId, DidYouMean, Flag, ParseError, ParseWarning, PositionalArg, Signature, Span,
+    Spanned, SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
 };
 
 use crate::parse_keywords::{
@@ -3520,6 +3520,13 @@ pub fn parse_signature_helper(working_set: &mut StateWorkingSet, span: Span) ->
                                         type_annotated,
                                     } => {
                                         working_set.set_variable_type(var_id.expect("internal error: all custom parameters must have var_ids"), syntax_shape.to_type());
+                                        if syntax_shape == SyntaxShape::Boolean {
+                                            working_set.warning(ParseWarning::DeprecatedWarning(
+                                                "--flag: bool".to_string(),
+                                                "--flag".to_string(),
+                                                span,
+                                            ));
+                                        }
                                         *arg = Some(syntax_shape);
                                         *type_annotated = true;
                                     }