Add the AI Playground (WIP) - #140

@felixarntz perhaps a docs page for this experiment like done in https://github.com/WordPress/ai/pull/155/changes#diff-f70665388390657902e73741dea03efcdadc4bd0ceb4432959f8903ae015c042?

@felixarntz do you think this could be ready for review/testing before end of month / roughly in time to get released around potentially updated WP 7.0 timing?

While I'm heavily in favor of this sort of functionality being available in the AI plugin, it seems like work here has stalled out and is becoming further out of sync with the plugin codebase that a new, clean approach may be best if this work is to be re-started. Thanks for the work here Felix, but for now I'm closing this PR as I "spring clean" the repo.

@jeffpaul That's fair. Though I believe it won't be a huge lift to refresh this against the latest AI plugin changes since it is largely decoupled and mostly directly interacts with the WP AI Client APIs, which haven't changed much.

For transparency: I still have this on my todo list, but time permitting. I plan to get back to this, hopefully within the next month. When I do, I'll reopen this PR because that'll still be much faster than rewriting from scratch. Please leave the branch around :)