Improve hardcoded navigation menus to use the correct site_url() rather than original value

[bobbingwide]

I've been creating the theme in s.b/wp56 so the url attribute in the navigation-link blocks is of the form https://s.b/wp56/permalink/

Here's the current footer menu.

<!-- wp:navigation {"orientation":"horizontal","itemsJustification":"center"} -->
<!-- wp:navigation-link {"label":"Blog page","url":"https://s.b/wp56/blog-page/"} /-->

<!-- wp:navigation-link {"label":"home-page","url":"https://s.b/wp56/"} /-->

<!-- wp:navigation-link {"label":"Trying to build Fizzie","url":"https://s.b/wp56/2020/10/trying-to-build-fizzie/"} /-->
<!-- /wp:navigation -->

When I implement the theme on blocks.wp-a2z, blocks.wp-a2z.org and other subdomains I'll want the menu items to point to the current site. I want the correct site url = as provided by site_url().

Proposed solution

In the absence of a solution delivered by Gutenberg, intercept the server side rendering using the same method as documented in #18.

eg

function fizzie_render_block_core_navigation_link( $attributes, $content, $block ) {
    $attributes = fizzie_nav_atts( $attributes );
    $html = gutenberg_render_block_core_navigation_link($attributes, $content, $block);
    return $html;
}

function fizzie_nav_atts( $attributes ) {
    $attributes['url'] = str_replace( 'https://s.b/wp56', site_url(), $attributes['url'] );
    return $attributes;
}

And while we're at it....
Apparently the logic already supports current-menu-item - see
WordPress/gutenberg#25575
so let's see if we can ensure that the current-menu-item class is set.

Looking at the current code, we need to set $attributes['id'] to the value of get_the_ID()
when the current URL is the same as the link's URL.

$is_active   = ! empty( $attributes['id'] ) && ( get_the_ID() === $attributes['id'] );
```

[bobbingwide]

The logic that uses get_the_ID() only works for singular content.

In wp_nav_menus(), WordPress uses a far more complicated function called _wp_menu_item_classes_by_context().
We can implement a much simpler function, currently called fizzie_fiddle_nav_atts().

In order to support setting the CSS class names for current menu item's ancestors I'll have to implement this logic for core/navigation rather than core/navigation-link.

[bobbingwide]

Fizzie v1.0.0 continues to apply overrides to the core/navigation-link block to replace hardcoded https://s.b/wp56 with site_url()