Reusable FontSettings and Theme customization

[mtias]

Follow up tasks from #5269.

Allow themes to specify the set of font size values. (Right now it defaults to pixels, but we might extend this.)

// $font_sizes = array(
//    'small' => 14,
//    'regular' => 16,
//    'large' => 36,
//    'larger' => 48,
// };

$font_sizes = get_theme_support( 'editor-font-sizes' );

Allow themes to disable custom font sizes (the slider and arbitrary input).

get_theme_support( 'disable-custom-font-sizes' );

Create reusable block component for font settings:

import FontSettings from '@wordpress/blocks/font-settings';

Here we could see whether we prefer a FontSize component or a full FontSettings with props for dropCap support, etc. Maybe FontSize scales better (like ColorPalette?).

[rchipka]

Related to #6651

Perhaps this component should be grouped into a BlockStyling component.

Also see the notes about how we should be doing class names and class injection to properly support extensibility.

[forsvunnet]

@mtias @jorgefilipecosta It doesn't look like this issue has been fully solved. It looks like the theme support to remove custom font sizes hasn't been implemented yet.

get_theme_support( 'disable-custom-font-sizes' );

Is there another way to remove/disable the font size control?