gen_stub: create VersionFlags class

This new class holds the logic for applying different flags based on the PHP
version, and replaces `VariableLike::addFlagForVersionsAbove()` (use
`VersionFlags::addForVersionsAbove()`) and `generateVersionDependentFlagCode()`
(use `VersionFlags::generateVersionDependentFlagCode()`).

Author: DanielEScherzer