Register Core Blocks on Init

[youknowriad]

closes #1097

Initially, I wanted to match the recommendations of the handbook (https://wordpress.org/gutenberg/handbook/blocks/writing-your-first-block-type/) for registering the Core Blocks server-side but at the moment the blocks don't have separate scripts so I just
updated the registration to move it to the init function instead.

Testing instructions

• Try the categories and recent posts blocks.

[aduth]

Looks good 👍

The function names (particularly dropdown script) aren't super clear, but I'm happy at least for dropping the Gutenberg codename 😄

In a merge proposal, unsure if we'd prefer to move toward registering all core blocks in a single location, though I like the isolation this currently gives in plugin form. We can revisit later.

[aduth]

I wanted to match the recommendations of the handbook for registering the Core Blocks server-side but at the moment the blocks don't have separate scripts

Related: #4841

Honestly, I'd hate for separate script bundles to be a blocker much longer for server registration, as it's hurting us the longer we wait for this to occur in developer confusion. I think we can go ahead and move all but the script handle to the server and tackle that separately.