Enable x-lang attribute for Servo engine

[MunishMummadi]

Description

This Pull Request includes the following changes:

style/properties/longhands/font.mako.rs

• Modified the helpers.predefined_type function call for -x-lang, adding support for gecko servo engines.

style/values/specified/font.rs

• Added implementation for XTextScale, providing functionality for handling text scaling.

refer at servo issue

[Loirooriol]

The patch looks good, but here in the description you are mentioning XTextScale which is unrelated?