enable flag value type checking (#11311)

# Description
Fixes: #11310

# User-Facing Changes
After the change, the following code will go to error:
```nushell
> def a [--x: int = 3] { "aa" }
> let y = "aa"
> a --x=$y
Error: nu::parser::type_mismatch

  × Type mismatch.
   ╭─[entry #32:2:1]
 2 │ let y = "aa"
 3 │ a --x=$y
   ·       ─┬
   ·        ╰── expected int, found string
   ╰────
```

src/tests/test_custom_commands.rs

@@ -73,6 +73,14 @@ fn custom_switch1() -> TestResult {
     )
 }
 
+#[test]
+fn custom_flag_with_type_checking() -> TestResult {
+    fail_test(
+        r#"def florb [--dry-run: int] { $dry_run }; let y = "3"; florb --dry-run=$y"#,
+        "type_mismatch",
+    )
+}
+
 #[test]
 fn custom_switch2() -> TestResult {
     run_test(
@@ -116,7 +124,7 @@ fn custom_flag1() -> TestResult {
         r#"def florb [
             --age: int = 0
             --name = "foobar"
-        ] { 
+        ] {
             ($age | into string) + $name
         }
         florb"#,