Adds number field

Number is probably the easiest one to add (a lot more work needed for checkbox, checkbox group and radio), but this adds a functional number field, I tested it with this and found no issues:

register_post_field( 'example_number_field', 'post',  array(
	'type'      => 'number',
	'label'     =>  __( 'Example Number Field', 'metadata-ui-examples' ),
	'min'       => 5,
	'max'      => 15,
	'step'      => 5
) );

Let me know if this is good, I'd like to work on other fields as well.