Add wc_has_block_template filter to WooCommerce Core

[tjcafferkey]

Description

As described in #4939 we need to update the return value of the following function

From

return is_readable(
    get_stylesheet_directory() . '/block-templates/' . $template_name . '.html'
);

To

return apply_filters( 'wc_has_block_template',
    is_readable(
        get_stylesheet_directory() . '/block-templates/' . $template_name . '.html'
    )
);

This will allow us to override this value from Woo Blocks to serve the block templates from Woo Blocks.

Acceptance Criteria

This will just require some regression testing to make sure everything is working as expected still.

• Given the same scenarios of rendering Block Templates over Legacy PHP Templates if they're available within the theme of a compatible Block Theme, everything should still work the same.