Plugin explicit flags (#11581)

# Description
#11492 fixed flags for builtin commands but I missed that plugins don't
use the same `has_flag` that builtins do. This PR addresses this.

Unfortunately this means that return value of `has_flag` needs to change
from `bool` to `Result<bool, ShellError>` to produce an error when
explicit value is not a boolean (just like in case of `has_flag` for
builtin commands. It is not possible to check this in
`EvaluatedCall::try_from_call` because

# User-Facing Changes
Passing explicit values to flags of plugin commands (like `--flag=true`
`--flag=false`) should work now.
BREAKING: changed return value of `EvaluatedCall::has_flag` method from
`bool` to `Result<bool, ShellError>`

# Tests + Formatting
Added tests and updated documentation and examples

tests/plugins/core_inc.rs

@@ -148,3 +148,16 @@ fn semversion_without_passing_field() {
         assert_eq!(actual.out, "0.1.4");
     })
 }
+
+#[test]
+fn explicit_flag() {
+    Playground::setup("plugin_inc_test_6", |dirs, _| {
+        let actual = nu_with_plugins!(
+            cwd: dirs.test(),
+            plugin: ("nu_plugin_inc"),
+            "'0.1.2' | inc --major=false --minor=true --patch=false"
+        );
+
+        assert_eq!(actual.out, "0.2.0");
+    })
+}