Script Modules: add the importmap's scopes key

[rosua]

WP_Script_Modules::print_import_map() prints the script modules, but it only prints the imports key.

As per described on the ​https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script/type/importmap#import_map_json_representation, <script type="importmap"> element can have another optional key: scopes and integrity.

There's no means for a plugin to add scopes to be printed.
So, besides the wp_register_script_module() function, we need a function to register the scopes.

The use case of using scopes are typical when we are using CDN providers such as ​https://generator.jspm.io/