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-protocol/src/engine/state_working_set.rs

@@ -6,7 +6,7 @@ use crate::ast::Block;
 use crate::{
     BlockId, Config, DeclId, FileId, Module, ModuleId, Span, Type, VarId, Variable, VirtualPathId,
 };
-use crate::{Category, ParseError, Value};
+use crate::{Category, ParseError, ParseWarning, Value};
 use core::panic;
 use std::collections::{HashMap, HashSet};
 use std::path::PathBuf;
@@ -27,6 +27,7 @@ pub struct StateWorkingSet<'a> {
     /// Whether or not predeclarations are searched when looking up a command (used with aliases)
     pub search_predecls: bool,
     pub parse_errors: Vec<ParseError>,
+    pub parse_warnings: Vec<ParseWarning>,
 }
 
 impl<'a> StateWorkingSet<'a> {
@@ -39,6 +40,7 @@ impl<'a> StateWorkingSet<'a> {
             parsed_module_files: vec![],
             search_predecls: true,
             parse_errors: vec![],
+            parse_warnings: vec![],
         }
     }
 
@@ -50,6 +52,10 @@ impl<'a> StateWorkingSet<'a> {
         self.parse_errors.push(parse_error)
     }
 
+    pub fn warning(&mut self, parse_warning: ParseWarning) {
+        self.parse_warnings.push(parse_warning)
+    }
+
     pub fn num_files(&self) -> usize {
         self.delta.num_files() + self.permanent_state.num_files()
     }