feat(extension): dispatch unknown top-level commands to installed extensions

[margaretjgu]

Part of #222.

When elastic <name> is called and <name> is not a built-in command, look up elastic-<name> in the extension registry and exec it, forwarding all remaining argv and the context env vars.

• Module at src/extension/runner.ts with a runExtension(ext, args, env) export
• In cli.ts, before program.parseAsync, check firstArg against the set of known built-ins; if unknown, attempt extension dispatch
• Exit with the extension process exit code
• If no matching extension found, let Commander handle the unknown command error normally

Depends on: #293, #294, #296