differentiating between --x and --x: bool (#10456)

# Description
Fixes: #10450 

This pr differentiating between `--x: bool` and `--x`

Here are examples which demostrate difference between them:
```nushell
def a [--x: bool] { $x };
a --x    # not allowed, you need to parse a value to the flag.
a        # it's allowed, and the value of `$x` is false, which behaves the same to `def a [--x] { $x }; a`
```

For boolean flag with default value, it works a little bit different to
#10450 mentioned:
```nushell
def foo [--option: bool = false] { $option }
foo                  # output false
foo --option         # not allowed, you need to parse a value to the flag.
foo --option true    # output true
```

# User-Facing Changes
After the pr, the following code is not allowed:
```nushell
def a [--x: bool] { $x }; a --x
```

Instead, you have to pass a value to flag `--x` like `a --x false`. But
bare flag works in the same way as before.

## Update: one more breaking change to help on #7260 
```
def foo [--option: bool] { $option == null }
foo
```
After the pr, if we don't use a boolean flag, the value will be `null`
instead of `true`. Because here `--option: bool` is treated as a flag
rather than a switch

---------

Co-authored-by: amtoine <stevan.antoine@gmail.com>

crates/nu-command/tests/commands/def.rs

@@ -180,3 +180,16 @@ fn def_default_value_should_restrict_implicit_type() {
     let actual2 = nu!("def foo2 [--x = 3] { $x }; foo2 --x 3.0");
     assert!(actual2.err.contains("expected int"));
 }
+
+#[test]
+fn def_boolean_flags() {
+    let actual = nu!("def foo [--x: bool] { $x }; foo --x");
+    assert!(actual.err.contains("flag missing bool argument"));
+    let actual = nu!("def foo [--x: bool = false] { $x }; foo");
+    assert_eq!(actual.out, "false");
+    let actual = nu!("def foo [--x: bool = false] { $x }; foo --x");
+    assert!(actual.err.contains("flag missing bool argument"));
+    // boolean flags' default value should be null
+    let actual = nu!("def foo [--x: bool] { $x == null }; foo");
+    assert_eq!(actual.out, "true");
+}