Validator: Initial implementation

[schlessera]

The AMP Toolbox should have a full PHP implementation of the AMP validator

• Generate PHP representation of validator spec => Add validator spec via code generation #100
• Implement validators that pass or fail validator spec rules
• Hook up https://github.com/ampproject/amphtml/tree/main/validator/testdata to verify parity with upstream
• Build CLI command to validate a URL or a piece of markup

[westonruter]

Interesting. The JavaScript implementation of the validator is being deprecated in favor of a WASM version which reuses the C++ implementation: ampproject/amphtml#36110

[schlessera]

Nice, now that I've ported the JS HTML Parser... :/

I'll go through the C++ code to see what needs adapting then.

[westonruter]

I believe they have had feature parity until now, so hopefully there's nothing that needs to change.