Suggestion: `DENO_KV_PATH` Environment Variable

I think it would be useful to have a built-in environment variable to override `Deno.openKv(path)` argument like `DENO_SERVE_ADDRESS`.

For example, like this:

- `DENO_KV_PATH` ... Possible value are same as `path` argument.

```sh
export DENO_KV_PATH='./bar.db'
```

```ts
// Both are overwritten with "./bar.db"
await Deno.openKv();
await Deno.openKv("./foo.db");
```