Publish a top-level meta package named antora

In addition to the individual component packages, publish a top-level meta package named antora. This package should depend on the CLI and site generator. Publishing this package has several advantages:

1. It claims the top-level antora name in the npm registry
2. It simplifies the installation of Antora (i.e., npm i antora)
3. It allows Antora to be used transiently with npx without needing to install it first (i.e., npx antora -v)

This package should provide the antora command which calls through to the antora command provided by @antora/cli. This is necessary for it to work across versions and configurations of npm.