Afform - Read afform defn from .aff.php files as well as .aff.json

Afform - Read afform defn from .aff.php files as well as .aff.json

By switching the file format, defn fields like `title` and `description` can be wrapped in ts().
This is intended for developers and extensions authors. They can generate these php files by calling `Afform.get`
with the api explorer, and copying the result (minus the "layout" field).
FormBuilder still saves files locally in `aff.json` format.
Overriding a packaged `.aff.php` form with a local `aff.json` works fine.