Enable short URLs in the playground (astral-sh#6383)

## Summary

This PR adds a [Workers
KV](https://developers.cloudflare.com/workers/runtime-apis/kv/)-based
database to the playground, which enables us to associate shared
snippets with a stable ID, which in turn allows us to generate short
URLs, rather than our existing extremely-long URLs.

For now, the URLs are based on UUID, so they look like
https://play.ruff.rs/a1c40d58-f643-4a3e-bc23-15021e16acef. (This URL
isn't expected to work, as the playground isn't deployed; it's only
included as an example.)

There are no visible changes in the UI here -- you still click the
"Share" button, which copies the link to your URL. There's no
user-visible latency either -- KV is very fast.

For context, with Workers KV, we provision a Workers KV store in our
Cloudflare account (`wrangler kv:namespace create "PLAYGROUND"`), and
then create a Cloudflare Worker that's bound to the KV store via the
`wrangler.toml`:

```toml
name = "db"
main = "src/index.ts"
compatibility_date = "2023-08-07"

kv_namespaces = [
  { binding = "PLAYGROUND", id = "672e16c4fb5e4887845973bf0e9f6021", preview_id = "0a96477e116540e5a6e1eab6d6e7523e" }
]
```

The KV store exists in perpetuity, while the Worker can be updated,
deployed, removed, etc. independently of the KV store. The Worker itself
has unfettered access to the KV store. The Worker is exposed publicly,
and just does some basic verification against the request host.

playground/README.md

@@ -4,11 +4,26 @@ In-browser playground for Ruff. Available [https://play.ruff.rs/](https://play.r
 
 ## Getting started
 
-- To build the WASM module, run `npm run build:wasm`
-    from the `./playground` directory.
-- Install TypeScript dependencies with: `npm install`.
-- Start the development server with: `npm run dev`.
+First, build the WASM module by running `npm run build:wasm` from the `./playground` directory.
 
-## Implementation
+Then, install TypeScript dependencies with `npm install`, and run the development server with
+`npm run dev`.
 
-Design based on [Tailwind Play](https://play.tailwindcss.com/). Themed with [`ayu`](https://github.com/dempfi/ayu).
+To run the datastore, which is based on [Workers KV](https://developers.cloudflare.com/workers/runtime-apis/kv/),
+install the [Wrangler CLI](https://developers.cloudflare.com/workers/wrangler/install-and-update/),
+then run `npx wrangler dev --local` from the `./playground/db` directory. Note that the datastore is
+only required to generate shareable URLs for code snippets. The development datastore does not
+require Cloudflare authentication or login, but in turn only persists data locally.
+
+## Architecture
+
+The playground is implemented as a single-page React application powered by
+[Vite](https://vitejs.dev/), with the editor experience itself powered by
+[Monaco](https://github.com/microsoft/monaco-editor).
+
+The playground stores state in `localStorage`, but can supports persisting code snippets to
+a persistent datastore based on [Workers KV](https://developers.cloudflare.com/workers/runtime-apis/kv/)
+and exposed via a [Cloudflare Worker](https://developers.cloudflare.com/workers/learning/how-workers-works/).
+
+The playground design is originally based on [Tailwind Play](https://play.tailwindcss.com/), with
+additional inspiration from the [Rome Tools Playground](https://docs.rome.tools/playground/).

playground/api/.dev.vars

@@ -0,0 +1,2 @@
+# See: https://developers.cloudflare.com/workers/wrangler/configuration/#environmental-variables
+DEV=1

playground/api/README.md

@@ -0,0 +1,5 @@
+# api
+
+Key-value store based on [Workers KV](https://developers.cloudflare.com/workers/runtime-apis/kv/).
+
+Used to persist code snippets in the playground and generate shareable URLs.