[intl] add support for determining whether a locale uses a RTL script

[stof]

Description

ICU4C has uloc_isRightToLeft to determine this.

It would be great to expose this in PHP. It is useful to manage the dir attribute in HTML based on the locale being used, instead of having to duplicate that knowledge in userland.

[devnexen]

Nice idea ! I ll have a better look at it soon.

[devnexen]

Added to master.