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>

src/tests/test_custom_commands.rs

@@ -67,22 +67,6 @@ fn do_rest_args() -> TestResult {
 
 #[test]
 fn custom_switch1() -> TestResult {
-    run_test(
-        r#"def florb [ --dry-run: bool ] { if ($dry_run) { "foo" } else { "bar" } }; florb --dry-run"#,
-        "foo",
-    )
-}
-
-#[test]
-fn custom_switch2() -> TestResult {
-    run_test(
-        r#"def florb [ --dry-run: bool ] { if ($dry_run) { "foo" } else { "bar" } }; florb"#,
-        "bar",
-    )
-}
-
-#[test]
-fn custom_switch3() -> TestResult {
     run_test(
         r#"def florb [ --dry-run ] { if ($dry_run) { "foo" } else { "bar" } }; florb --dry-run"#,
         "foo",
@@ -90,7 +74,7 @@ fn custom_switch3() -> TestResult {
 }
 
 #[test]
-fn custom_switch4() -> TestResult {
+fn custom_switch2() -> TestResult {
     run_test(
         r#"def florb [ --dry-run ] { if ($dry_run) { "foo" } else { "bar" } }; florb"#,
         "bar",

src/tests/test_parser.rs

@@ -440,7 +440,7 @@ fn string_escape_interpolation2() -> TestResult {
 #[test]
 fn proper_rest_types() -> TestResult {
     run_test(
-        r#"def foo [--verbose(-v): bool, # my test flag
+        r#"def foo [--verbose(-v), # my test flag
                    ...rest: int # my rest comment
                 ] { if $verbose { print "verbose!" } else { print "not verbose!" } }; foo"#,
         "not verbose!",