Change FontHinting::should_hint to pub

[ickshonpe]

Objective

• A trivial helper function like FontHinting::should_hint should be pub if it exists.
• should_hint doesn't seem very idiomatic and doesn't match the names of the enum's variants.

Solution

• Renamed FontHinting::should_hint to is_enabled.
• Changed FontHinting::is_enabled from pub(crate) to pub.
• Added the required doc comment now its public.