New xml format (#7947)

# Description

Changes old `from xml` `to xml` data formats. See #7682 for reasoning
behind this change.
Output is now a series of records with `tag`, `attributes` and `content`
fields.

Old:

![image](https://user-images.githubusercontent.com/17511668/224508728-92d37c1f-ebac-4d5c-924d-bebd60f5cf85.png)
New:

![image](https://user-images.githubusercontent.com/17511668/224508753-a2de338a-ff2a-41e0-bbc1-ccc07a1d00ce.png)


# User-Facing Changes

New output/input format, better error handling for `from xml` and `to
xml` commands.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/tests/commands/open.rs

@@ -179,7 +179,18 @@ fn parses_json() {
 fn parses_xml() {
     let actual = nu!(
         cwd: "tests/fixtures/formats",
-        "open jonathan.xml | get rss.children.channel.children | get 0.3.item.children | get 3.link.children.0"
+        pipeline(r#"
+            open jonathan.xml
+            | get content
+            | where tag == channel
+            | get content
+            | flatten
+            | where tag == item
+            | get content
+            | flatten
+            | where tag == guid
+            | get content.0.content.0
+        "#)
     );
 
     assert_eq!(

crates/nu-command/tests/format_conversions/xml.rs

@@ -8,7 +8,15 @@ fn table_to_xml_text_and_from_xml_text_back_into_table() {
             open jonathan.xml
             | to xml
             | from xml
-            | get rss.children.channel.children.0.3.item.children.guid.4.attributes.isPermaLink
+            | get content
+            | where tag == channel
+            | get content
+            | flatten
+            | where tag == item
+            | get content
+            | flatten
+            | where tag == guid
+            | get 0.attributes.isPermaLink
         "#
     ));