Ship javascript, not binaries

[twitchard]

1. Stop using commands from Bun's standard library -- use the nodejs standard library instead.
2. Don't use bun to compile a standalone binary. Instead, put #!/usr/bin/env node at the top of index.ts and ship an index.js with the bundled Javascript for the entire CLI.

Downside of this: bun's startup time is faster so now the CLI will boot up a little slower when the user types a command.
Upside of this: we are essentially opting out of handling platform cross-compatibility ourselves, all we now rely upon is the user having a supported node binary somewhere on their system (which is pretty likely if they installed us via npm install @humeai/cli)

Tested via
npm run prebuild && dist/index.js tts "Hello" on both Linux and Mac.