Add WebPack build process and load JS in editor screen

[johnwatkins0]

This will resolve #12.

Description of the Change

• Sets up a simple ESLint config using the 10up standard and based on other recent 10up plugins.
• Creates a basic Webpack config with babel-loader and a .babelrc file with env and react presets. It builds a file from src/js into dist.
• Creates watch and build scripts using Webpack in package.json
• Creates an assets.php file to enqueue admin scripts and styles, with the necessary logic needed to determine whether to load the new JS assets or the existing JS/CSS created for the pre-5.0 version of the autotweet feature.

Not in this PR

• This contains no working JS beyond a console.log statement for verification. For core JS dependencies, I made an educated guess of wp-plugins and wp-edit-post, understanding this may need to evolve.
• This doesn't update the existing JS/CSS built for pre-5.0 WP. The JS will need to be updated later (at least to accommodate the REST endpoint coming with Add REST API endpoint to replace wp_ajax_tenup_auto_tweet [3 hrs.] #14).

Verification Process

Build the JS with npm run watch or npm run build. Verify that the correct JS file loads when the block editor is loaded with a supported post type; verify that the older JS/CSS files load when the classic editor is loaded with a supported post type.

[johnwatkins0]

Thanks, @adamsilverstein . I removed that test console.log.

[adamsilverstein]

Fantastic!