same as harfbuzz hb_script_get_horizontal_direction.
We just hard-code the set of RTL script here, as it doesn't change often anyway.
The function is just syntactic sugar as it all does is basically looking up the
constant RTL_SCRIPTS set.
It's nice to have it here in a central place alongside 'script', 'script_name', etc.
`script_code` does the reverse of `script_name`: it takes a long
script name and returns a 4-letter script code.
Both `script_name` and `script_code` raise KeyError by default,
but can optionally return a default value instead.
