xpath command for nushell (#2656)

* xpath prototype

* new xpath engine is finally working

* nearly there

* closer

* working with list, started to add test, code cleanup

* broken again

* working again - time for some cleanup

* cleaned up code, added error handling and test

* update example, fix clippy

* removed commented char

crates/nu-cli/tests/commands/xpath.rs

@@ -0,0 +1,41 @@
+use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn position_function_in_predicate() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "<?xml version="1.0" encoding="UTF-8"?><a><b/><b/></a>" | from xml | to xml | xpath "count(//a/*[position() = 2])"
+        "#
+    ));
+
+    assert_eq!(actual.out, "1.0000");
+}
+
+#[test]
+fn functions_implicitly_coerce_argument_types() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "<?xml version="1.0" encoding="UTF-8"?><a>true</a>" | from xml | to xml | xpath "count(//*[contains(., true)])"
+        "#
+    ));
+
+    assert_eq!(actual.out, "1.0000");
+}
+
+#[test]
+fn find_guid_permilink_is_true() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open jonathan.xml
+            | to xml
+            | xpath '//guid/@isPermaLink'
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}