support filesize arguments in random binary/chars (#14068)

Closes #13920

# User-Facing Changes

`random binary` and `random chars` now support filesize arguments:

```nushell
random binary 1kb
random chars --length 1kb
```

crates/nu-command/tests/commands/random/chars.rs

@@ -9,3 +9,13 @@ fn generates_chars_of_specified_length() {
     let result = actual.out;
     assert_eq!(result, "15");
 }
+
+#[test]
+fn generates_chars_of_specified_filesize() {
+    let actual = nu!(r#"
+        random chars --length 1kb | str stats | get bytes
+        "#);
+
+    let result = actual.out;
+    assert_eq!(result, "1000");
+}