Register editor script/style with block instead of using enqueue_block_editor_assets

[westonruter]

Fixes #689

Previously the plugin was enqueuing the editor script and style manually at the enqueue_block_editor_assets action. This broke as of Gutenberg 14.8 in which an editor-canvas iframe was introduced to encapsulate the edited blocks. The issue is fixed by registering the script and style via the editor_script_handles and editor_style_handles props of WP_Block_Type, respectively, which allows WordPress to know specifically which assets need to be enqueued in the editor-canvas.