Plugin: Populate demo content by default content filters

[aduth]

This pull request seeks to move handling of the demo content initialization from gutenberg_editor_scripts_and_styles to separate filter handlers in a new file dedicated for demo supports (one of the few PHP extensions Gutenberg will offer atop core from the initial 5.0 release). The reason for this change is to facilitate the removal of gutenberg_editor_scripts_and_styles. Core doesn't otherwise allow for filtering of the $initial_edits it generates for editor initialization, but these can be alternatively provided through the default_content and default_title filters run by get_default_post_to_edit (the function responsible for generating a new post to edit).

Testing instructions:

Verify that the Gutenberg demo content is shown for a new post created via Gutenberg > Demo.

Verify that Gutenberg demo content is not shown for a new post created via Posts > Add New.

Verify that Gutenberg content is not considered even if manually adding a &gutenberg-demo query argument to the URL of any edited post.

[youknowriad]

Works well on my testing 👍

[youknowriad]

Works well on my testing 👍