DECSWT and DECSIN escape sequences are incorrect

### Description

The `DECSWT` function produces an escape sequence starting with `OSC 2;1`, because you're just prepending `1;` on the name passed to the `SetWindowTitle` function, which is an `OSC 2` sequence. See here:
https://github.com/charmbracelet/x/blob/f2fb44ab31459eb3be0bfab80478bd476edee51a/ansi/title.go#L38-L40

But it really should be starting with `OSC 21`, so something more like this:
```cpp
return "\x1b]21;" + name + "\x1b\\"
```

Similarly, the `DECSIN` function produces an escape sequence starting with `OSC 2;L`, because again you're prepending `L;` on the name passed to `SetWindowTitle`. See here:
https://github.com/charmbracelet/x/blob/f2fb44ab31459eb3be0bfab80478bd476edee51a/ansi/title.go#L46-L48

When it should be starting with `OSC 2L`, so something more like this:
```cpp
return "\x1b]2L;" + name + "\x1b\\"
```

Also note that the string terminator is `ST` (which is `ESC \`) and not a `BEL` control (`\x07`). Most modern terminal emulators will accept `BEL` as a terminator, but a real VT520/VT525 will not, so everything following that `BEL` will still be parsed as part of the window title until it receives some form of `ESC`.

You can confirm the correct sequences for these functions in the VT520/VT525 programmer's reference (EK-VT520-RM) found here: https://vt100.net/dec/ek-vt520-rm.pdf

And if you don't have access to a real VT520 or VT525 to test on, I know Windows Terminal at least supports the `DECSWT` sequence. I don't think it supports `DECSIN` though.

### Version

master

### Environment

any

Start in ansi/title.go at the DECSWT and DECSIN definitions linked in the issue. Compare their emitted sequences with the cited VT520/VT525 reference, including the ST terminator. Add focused regression coverage for both functions and run the relevant Go tests.

Written by the indexing model from the issue text.