define WP_ENVIRONMENT_TYPE: 'local' in playground

[ashfame]

Required to expose dev tooling's UI in WP admin.

@psrpinto @akirk

This works for now, but before releasing the extension, I think we would need a different value for WP_ENVIRONMENT_TYPE or it not be defined since default value is production, so that dev tooling UI is not exposed to the user. Perhaps a different set of steps for prod build, if we have the need?

I can create a new issue to track this, if we all agree that this UI should not be exposed to the end user.

[psrpinto]

I think it would make sense to, in the future, have a isDev (or similar) somewhere in the frontend code, that can be set through the build tooling (not sure how we would do this). Then we can modify the behaviour of the code according to the value of isDev, e.g. expose or not expose window.trywp, or set the value of WP_ENVIRONMENT_TYPE.