Move show_in_rest to meta in the registration process

[gziolo]

• Part of Determine what goes in Ability meta #106

Move show_in_rest to meta in the registration process.

Example:

// Ability that does not show in REST.
wp_register_ability(
	'test/not-show-in-rest',
	array(
		'label'               => 'Hidden from REST',
		'description'         => 'It does not show in REST.',
		'execute_callback'    => static function (): int {
			return 0;
		},
		'permission_callback' => '__return_true',
		'meta'                => array(
			'show_in_rest' => false,
		),
	)
);

I also included the proposal in 8f937a6 to change the default value to true, so all abilities are exposed in REST API by default. The decision was made in #107 (comment) to keep show_in_rest defaulting to false.

I also replaced show_in_rest() method with a more generalized helper get_meta_item(). Example usage:

$ability->get_meta_item( 'non_existing' ); // returns null
$ability->get_meta_item( 'non_existing', 'default_value' ); // returns 'default_value'
$ability->get_meta_item( 'show_in_rest' ); // returns false

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.27%. Comparing base (76f9cea) to head (207146e).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #107      +/-   ##
============================================
+ Coverage     86.17%   86.27%   +0.10%     
- Complexity      110      111       +1     
============================================
  Files            16       16              
  Lines           803      809       +6     
  Branches         86       86              
============================================
+ Hits            692      698       +6     
  Misses          111      111              

Flag	Coverage Δ
javascript	92.62% <ø> (ø)
unit	83.95% <100.00%> (+0.16%)	⬆️

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Co-authored-by: gziolo <[email protected]>
Co-authored-by: JasonTheAdams <[email protected]>
Co-authored-by: felixarntz <[email protected]>
Co-authored-by: justlevine <[email protected]>

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[JasonTheAdams]

Looking mostly good, just one question on the default, @gziolo.

[gziolo]

The decision was made in #107 (comment) to keep show_in_rest defaulting to false. I addressed all other feedback, too.