Webfonts API: Return font family slug when registering a webfont

[zaguiini]

What?

Return the font family slug when registering a webfont.

Why?

There are cases where you want to be aware of the font family slug. One example is https://github.com/Automattic/jetpack/pull/23810/files#diff-67965f8a19767c6775dd0d0949ed5830c22bda270ec3c1835f801649e98596c2R70.

Returning the slug removes the need to call WP_Webfonts::get_font_slug after registering a webfont.

How?

Simply returning the $slug variable from the register_webfont method is enough.

Testing Instructions

You should check that the font family slug is returned after calling wp_register_webfont and wp_register_webfonts.

[jeyip]

Testing

Requirements

• wp_register_webfont returns the registered slug string
• wp_register_webfonts returns the array of registered slug strings

// I tested this PR by error logging the output of wp_register_webfont and 
// wp_register_webfonts in the functions.php file of the currently active theme
$slug = wp_register_webfont(
		array(
			'font-family'  => 'Roboto',
			'font-style'   => 'normal',
			'font-stretch' => 'normal',
			"font-weight" => "900",
			'src'          => array( 'file:./assets/fonts/Roboto-Regular.ttf' ),
		)
	);

// Outputs 'roboto'
error_log( $slug );

$slugs = wp_register_webfonts(array(
		array(
			'font-family'  => 'Roboto',
			'font-style'   => 'normal',
			'font-stretch' => 'normal',
			"font-weight" => "900",
			'src'          => array( 'file:./assets/fonts/Roboto-Regular.ttf' ),
		)
	));

// Outputs [ 'roboto' ]
error_log( $slugs );

[hellofromtonya]

LGTM 👍