Make ++ operator work with strings and binary values (#8017)

This PR makes `++` (the append operator) work with strings and binary
values. Can now do things like:

```bash
〉"a" ++ "b"
ab
〉0x[01 02] ++ 0x[03]
Length: 3 (0x3) bytes | printable whitespace ascii_other non_ascii
00000000:   01 02 03                                             •••
```

Closes #8015.

crates/nu-command/tests/commands/math/mod.rs

@@ -473,6 +473,8 @@ fn compound_where_paren() {
     assert_eq!(actual.out, r#"[{"a": 2,"b": 1},{"a": 2,"b": 2}]"#);
 }
 
+// TODO: these ++ tests are not really testing *math* functionality, maybe find another place for them
+
 #[test]
 fn adding_lists() {
     let actual = nu!(
@@ -519,3 +521,25 @@ fn adding_tables() {
     ));
     assert_eq!(actual.out, "[{a: 1, b: 2}, {c: 10, d: 11}]");
 }
+
+#[test]
+fn append_strings() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            "foo" ++ "bar"
+        "#
+    ));
+    assert_eq!(actual.out, "foobar");
+}
+
+#[test]
+fn append_binary_values() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            0x[01 02] ++ 0x[03 04] | to nuon
+        "#
+    ));
+    assert_eq!(actual.out, "0x[01020304]");
+}