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-parser/src/type_check.rs

@@ -84,6 +84,8 @@ pub fn math_result_type(
                     }
                 }
                 (Type::Table(a), Type::Table(_)) => (Type::Table(a.clone()), None),
+                (Type::String, Type::String) => (Type::String, None),
+                (Type::Binary, Type::Binary) => (Type::Binary, None),
                 (Type::Any, _) | (_, Type::Any) => (Type::Any, None),
                 _ => {
                     *op = Expression::garbage(op.span);