Block support: Add server-side processing for anchor

[t-hamano]

Trac ticket: https://core.trac.wordpress.org/ticket/64449

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[westonruter]

It doesn't seem this function has any test coverage?

[t-hamano]

Tests added in 8fee9e1.

[westonruter]

This looks good to me. A couple other tiny nits/suggestions which aren't necessary for merge.

[westonruter]

Since the value can only be null or array, this would seem more precise (although I know other block supports are doing this as well):

Suggested change

	if ( ! $block_type->attributes ) {
	if ( ! isset( $block_type->attributes ) ) {

This is slightly better from a type perspective, because with PHPStan's strict rules, specifically booleansInConditions, using anything other than a bool in the condition is flagged as an issue. Granted, WordPress core does this all over the place, but maybe we can start moving away from it with Core-64238 and the like.

[westonruter]

Could be combined. Not critical.

Suggested change

	if ( ! isset( $block_attributes['anchor'] ) || ! is_string( $block_attributes['anchor'] ) ) {
	return array();
	}
	if ( '' === $block_attributes['anchor'] ) {
	return array();
	}
	if ( ! isset( $block_attributes['anchor'] ) || ! is_string( $block_attributes['anchor'] ) || '' === $block_attributes['anchor'] ) {
	return array();
	}