docs(README): update Homebrew Cask and scoop install tip for all languages (#1880)

## Description

Adds the official Homebrew Cask install command for macOS to the README
installation tip.

This lets macOS users install AIRI directly with:

```sh
brew install --cask airi
```

without needing to add a custom Homebrew tap.

## Linked Issues

Related to #866

Related to #1431

## Additional Context

AIRI is now available as a Homebrew Cask, so the previous tap-based
installation discussion can be resolved with the standard cask install
flow.