Convert block validation script to a module, and bundle it with block toggle module

[kienstra]

AC1. Convert amp-block-validation.js to an ES6 module
AC2. Create a new file that will import both this module and amp-block-editor-toggle.js (though that might have since been renamed). This can be similar to blocks/index.js.
AC3. Add that new file with the import statements to webpack.config.js, so it will be compiled with npm run dev
AC4. Ensure that the compiled files have sourcemaps.

This should probably wait for the work on amp-block-validation.js in #1293

[kienstra]

Hi @postphotos,
If it's alright, I opened this issue with basic ACs. Feel free to edit this if you'd like.

This arose from the work on #1275 (comment).

[hellofromtonya]

@kienstra PR #1293 is now merged. You have a green light to convert the amp-block-validation.js script into an E6 module.

[kienstra]

Thanks, @hellofromtonya! I'm not sure if I'll work on this issue soon, though. In the meantime, this is free for anyone to pick up.

[kienstra]

Adding Another Script To The Requirements

Per this conversation, please also convert amp-invalid-url-post-edit-screen.js to an ES6 module.

[westonruter]

I'm going to close this because this will come automatically once we refactor the admin screens to use JS-driven interfaces as opposed to the PHP-driven ones we've largely tried to re-use from what WordPress provides.