Replace `__unstableIsBlockBasedTheme` in block-editor api with core data API

[t-hamano]

Related to #53137

What problem does this address?

Currently, to determine if a activetheme is a block-based theme, we use the Block Editor API as follows:

import { store as blockEditorStore } from '@wordpress/block-editor';

const isBlockBasedTheme = useSelect( ( select ) => {
    return select( blockEditorStore ).getSettings().__unstableIsBlockBasedTheme;
}, [] );

However, if the current page does not load a block editor, this API returns undefined.

What is your proposed solution?

In order to accurately determine if it is a block-based theme, it would be good to replace it with a core data API, as suggested in this comment.

import { store as coreStore } from '@wordpress/core-data';

const isBlockBasedTheme = useSelect( ( select ) => {
    return select( coreStore ).getCurrentTheme().is_block_theme;
}, [] );

[HILAYTRIVEDI]

I am taking this issue 😄

[Mamaduka]

Thanks, @HILAYTRIVEDI.

The change needs to happen in WP Core and WP_REST_Themes_Controller.

@t-hamano, I think this is probably not a "Good First Issue" task.