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>

src/tests/test_custom_commands.rs

@@ -146,6 +146,13 @@ fn custom_flag2() -> TestResult {
     )
 }
 
+#[test]
+fn deprecated_boolean_flag() {
+    let actual = nu!(r#"def florb [--dry-run: bool, --another-flag] { "aaa" };  florb"#);
+    assert!(actual.err.contains("Deprecated"));
+    assert_eq!(actual.out, "aaa");
+}
+
 #[test]
 fn simple_var_closing() -> TestResult {
     run_test("let $x = 10; def foo [] { $x }; foo", "10")