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-std/std/help.nu

@@ -100,7 +100,7 @@ def "nu-complete list-externs" [] {
 
 def build-help-header [
     text: string
-    --no-newline (-n): bool
+    --no-newline (-n)
 ] {
     let header = $"(ansi green)($text)(ansi reset):"