OpenStatSpec PHP v0.5.0

What's Changed

• chore: adopt OpenStatSpec SPSS codec 3.0.3 by @TonisOrmisson in #3
• Document codec 3.0.3 and transformation parity boundary by @TonisOrmisson in #4
• Add basic variable transformations by @TonisOrmisson in #5
• release: pin PHP adapter to specification v0.3.0 by @TonisOrmisson in #6
• Add PHP in-place transformation service evidence by @TonisOrmisson in #7
• Prepare OpenStatSpec PHP v0.5.0 by @TonisOrmisson in #8

Full Changelog: v0.4.0...v0.5.0