Support Arabic fallback shaping

Test text:
ح ب ا حبا
It consists of two parts (separated with a space):
حبا
ح ب ا
The second part is basically the first part with every character delimited with space (in order to cancel ligatures).

Related buffer config:
```rs
buffer.set_direction(rustybuzz::Direction::RightToLeft);
buffer.set_language(rustybuzz::Language::from_str("ar").unwrap());
buffer.set_script(rustybuzz::script::ARABIC);
```

Test case 1 (unifont-15.0.01.ttf)
Shaped glyph id:
harfbuzz `56721 56725 56237 35 56720 35 56722 35 56740`
rustybuzz `1578 1579 1584 35 1578 35 1579 35 1584`

So firstly, I had tested with unifont, and rustybuzz failed to handle the ligatures. I thought I must had got something wrong, but then I tested with another font.

Test case 2 (HONORSansArabicUI-B.ttf)
Shaped glyph id:
harfbuzz `7 33 97 1 5 1 30 1 93`
rustybuzz `7 33 97 1 5 1 30 1 93`

And the result is correct and conforms to that of harfbuzz.

There might be something off with rustybuzz.