Avoid confusion around blockTypes/allowedBlockTypes/enabledBlockTypes setting

[noisysocks]

Quick cleanup that does two things.

First, it makes the enabledBlockTypes argument in getInserterItems and getFrecentInserterItems non-optional. I regret making this optional since it implies that not providing this argument is a sensible thing to do, when in reality it would cause subtle bugs, e.g. a user could insert a block that has been disabled via the allowed_block_types filter.

Since selectors are exposed publicly now, I added a deprecation warning so as to avoid breaking backwards compatibility. Let me know if you think that's overkill.

Secondly, I noticed that we refer to this array of allowed block types throughout the codebase as either blockTypes, allowedBlockTypes or enabledBlockTypes. I've renamed these variables so that we are always referring to this array as allowedBlockTypes, which matches the name of the filter.

How I tested this

1. Create a post, insert some regular and shared blocks. There should be no warnings in the console
2. Add the following to the top of lib/load.php:

add_filter( 'allowed_block_types', function() {
	return [ 'core/paragraph', 'core/image', 'core/quote' ];
} );

3. Create a post. Test that only paragraphs, images and quotes can be inserted via the inserter and quick inserter (the thing that appears to the right of Write your story).

[noisysocks]

cc. @brandonpayton

[gziolo]

Unrelated to this PR, but I wanted to double check if we need to open another issue. Shouldn't this logic verify whether core/block isn't allowed, but also referencedBlockType.name?

[noisysocks]

In my opinion, disabling core/block should prevent you from inserting all shared blocks, but I could be persuaded otherwise. Let's discuss in #5893.

[gziolo]

Since this line is updated. It can be simplified to:

hasSupportedBlocks: ! isEmpty( allowedBlockTypes )

[noisysocks]

Very confusingly for us PHP folks, _.isEmpty() doesn't work the same way as empty():

$ node
> var _ = require('lodash');
> _.isEmpty(true)
true
> _.isEmpty(false)
true

[gziolo]

Sharing for a reference from https://wordpress.org/gutenberg/handbook/extensibility/extending-blocks/#hiding-blocks-from-the-inserter:

Hiding blocks from the inserter

On the server, you can filter the list of blocks shown in the inserter using the allowed_block_types filter. you can return either true (all block types supported), false (no block types supported), or an array of block type names to allow.

I like the changes introduced in this PR 👍

[gziolo]

Both depreciation messages should be also added to https://github.com/WordPress/gutenberg/blob/master/docs/deprecated.md for better visibility.