Add npm to the README install options

[PaulAsjes]

npm distribution is live — all six packages publish at 1.1.0 — so the README should list it alongside Homebrew, Scoop and the shell installers.

### npm (macOS / Linux / Windows)

npm install -g @elevenlabs/cli

Installs the same binary through a thin launcher, picking the right build for
your platform. Use `npx @elevenlabs/cli <command>` to run it once without
installing it globally.

Placed after Scoop so the three package managers sit together and the raw installers stay below them, matching the ordering in the docs snippet (elevenlabs-dx#2651).

Why the wording

"the same binary" is now literally true rather than a nicety: since #134 the npm packages repackage the archives cargo-dist built for the tag, verified with gh attestation verify, instead of compiling their own. Before that they were a separate build that selected a different TLS backend on macOS and Windows.

Verified

$ npm install @elevenlabs/cli   ->  added 2 packages
$ elevenlabs --version          ->  elevenlabs 1.1.0
$ npx @elevenlabs/cli@1.1.0 models list --dry-run   ->  works

The npx line is checked with a real subcommand, not just --version.

Note

This README ships inside the @elevenlabs/cli npm package as of #135, so it is also what npmjs.com will render for the package from 1.2.0 onward. The 1.1.0 page stays blank until then — that tarball predates the fix and npm versions are immutable.

The table of contents lists only ## headings, so it needs no change.