Deno v2.6.9: Markdown docs, better checksums, Node compat fixes v2.6.9

Shipped markdown support in doc generation, checksum verification for upgrades, and a heap of Node.js compatibility fixes (worker threads, fs.writeFile, .only tests, zstd compression). Windows tunnel fix and improved error messages for troubleshooting included. Lowkey one of those patches where the little things add up to actually being useful.