chore: require a distributed KV database via env var (#35671)
A platform that attaches Deno KV databases out-of-band (for example Deno
Deploy) has no way today to make a missing database obvious. When no
database is attached, `Deno.openKv()` either is not exposed at all, so
calling it reports the confusing "Deno.openKv is not a function", or it
silently falls back to a per-isolate in-memory store whose data never
persists and is not shared between instances.
This adds a `DENO_KV_REQUIRES_DISTRIBUTED_DATABASE` env var that
controls what happens when `Deno.openKv()` would resolve to a local or in-memory
store rather than a distributed (remote) database. Set to `error`, the
KV API is force-exposed even without `--unstable-kv` and the open
rejects with a clear "no KV database is attached" message. Set to `warn`, the
local fallback is still allowed but a warning is logged once. When the
var is unset, behavior is unchanged. A distributed database is one
served over http(s), matching the remote KV backend.
Co-authored-by: Bert Belder <bertbelder@gmail.com>
