docs: add Deno to install instructions

👋 Bartek from the Deno team here. The install block lists the usual package managers. Since Deno is a drop-in replacement for npm these days, this adds a Deno entry in parity: ```sh deno add vee-validate ``` Docs-only change. Totally fine to close this if you'd rather not. Happy to adjust. _Disclosure: I work on Deno, so I have an interest here. The goal is parity with the package managers you already list, not promotion. This PR was prepared with AI assistance under human supervision: I review every change myself and personally respond to any comments or review feedback._