Define Post Model Terms Fluently

[srtfisher]

Extends #276
Docs https://github.com/alleyinteractive/mantle-docs/pull/21/files

Allows post models to add/set/delete/update terms using a fluent format.

The Post model support interacting with terms through relationships or through the model directly. The model supports multiple methods to make setting terms on a post simple:

$category = Category::whereName( 'Example Category' )->first();

// Save the category to a post.
$post = new Post( [ 'title' => 'Example Post' ] );

// Also supports an array of IDs or WP_Term objects.
$post->terms->category = [ $category ];

$post->save();

// Read the tags from a post.
$post->terms->post_tag // Term[]

Terms can also be set when creating a post (specifying the taxonomy is optional):

$post = new Post( [
	'title' => 'Example Title',
	'terms' => [ $category ],
] );

$post = new Post( [
	'title' => 'Example Title',
	'terms' => [
		'category' => [ $category ],
		'post_tag' => [ $tag ],
	],
] );

[anubisthejackle]

Looks good. I would confirm my suspicions about the logic concern in one of the assertions before merging, but everything else seems good to go.

🎸

[anubisthejackle]

I know this has already been merged, but I wanted to call out that without PHPUnit::assertNotInstanceOf we no longer have an assertion if $term is null. That was the intent behind that line in my previous suggestion.