feat(unstable): append commit versionstamp to key

[losfair]

const kv = await Deno.openKv();
const { versionstamp } = await kv.set(["logs", kv.commitVersionstamp()], "a");
const { value } = await kv.get(["logs", versionstamp]); // value === "a"

[igorzi]

what if commitVersionstamp() is used as not the last part of the key? or if its used with operation other than "set"? seems like we should throw in that case?

[losfair]

Native code checks for that as part of deserialization, and throws

[losfair]

Added to the test