Add -i flag back to get and select (#8488)

https://github.com/nushell/nushell/pull/8379 removed the `-i` flag from
`get` and `select` because the new `?` functionality covers most of the
same use cases. However, https://github.com/nushell/nushell/issues/8480
made me realize that `-i` is still useful when dealing with cell paths
in variables.

This PR re-adds the `-i` flag to `get` and `select`. It works by just
marking every member in the cell path as optional, which will behave
_slightly_ differently than `-i` used to (previously it would suppress
any errors, even type errors) but IMO that's OK.

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

@@ -241,3 +241,16 @@ fn get_does_not_delve_too_deep_in_nested_lists() {
 
     assert!(actual.err.contains("cannot find column"));
 }
+
+#[test]
+fn ignore_errors_works() {
+    let actual = nu!(
+        cwd: ".",
+        r#"
+        let path = "foo";
+        {} | get -i $path | to nuon
+        "#
+    );
+
+    assert_eq!(actual.out, "null");
+}

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

@@ -256,3 +256,16 @@ fn select_failed4() {
 
     assert!(actual.err.contains("Select can't get the same row twice"));
 }
+
+#[test]
+fn ignore_errors_works() {
+    let actual = nu!(
+        cwd: ".",
+        r#"
+        let path = "foo";
+        [{}] | select -i $path | to nuon
+        "#
+    );
+
+    assert_eq!(actual.out, "[[foo]; [null]]");
+}