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-cli/src/eval_cmds.rs

@@ -35,6 +35,10 @@ pub fn evaluate_commands(
         let mut working_set = StateWorkingSet::new(engine_state);
 
         let output = parse(&mut working_set, None, commands.item.as_bytes(), false);
+        if let Some(warning) = working_set.parse_warnings.first() {
+            report_error(&working_set, warning);
+        }
+
         if let Some(err) = working_set.parse_errors.first() {
             report_error(&working_set, err);
 

crates/nu-cli/src/util.rs

@@ -220,6 +220,10 @@ pub fn eval_source(
             source,
             false,
         );
+        if let Some(warning) = working_set.parse_warnings.first() {
+            report_error(&working_set, warning);
+        }
+
         if let Some(err) = working_set.parse_errors.first() {
             set_last_exit_code(stack, 1);
             report_error(&working_set, err);