AI: Introduce `wp_supports_ai()` function to control LLM-related features.

This includes a `WP_AI_SUPPORT` constant and a `wp_supports_ai` filter.

When false,
- `_wp_connectors_get_provider_settings()` will return an empty array (short-circuiting the other functionality).
- `WP_AI_Client_Prompt_Builder()` will short-circuit the construction with a `WP_Error()`. `wp_ai_client_prompt()` will still return an instance, to allow for fluidity to remain intact.

Priority: `WP_AI_SUPPORT` > `add_filter( 'wp_supports_ai', ...) > (default: true)` 

Follow-up to [61943], [61749], [61943].

Props justlevine, westonruter, gziolo, flixos90, romainmrhenry, ahortin, chrismcelroyseo, SergeyBiryukov.
See #64706.

git-svn-id: https://develop.svn.wordpress.org/trunk@62067 602fd350-edb4-49c9-b593-d223f7449a82