Use native Deno API to start subprocess

[link2xt]

Deno has its own API: https://docs.deno.com/api/deno/subprocess

Currently the bot is using Node.js API:

echo/deno/index.js

Line 5 in f0a8629

import process from "node:process";

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In #74 I moved Deno example out of Node.js folder, but it is using Node.js packages and Node.js APIs for everything thanks to some Node.js compatibility Deno 2 introduced. This issue is part of migrating Deno example away from this Node.js compatibility.

See also #76