chore: parameterize Node.js 22 in .node-version file

[MikeMcC399]

Change

Add a .node-version file to the root of the repo with Node.js contents 22 to

• set the recommended Node.js version to 22 which is equivalent to the latest Node.js Active LTS release
• allow Node.js version managers, such as n, to more easily synchronize the required version on a local system. node-version-usage contains a list of such Node.js version managers
• allow the workflow .github/workflows/main.yml to use the defined version through actions/setup-node option node-version-file

Update the readme with information on the meaning of .node-version.

Related

This change was already implemented in the companion repo https://github.com/cypress-io/component-testing-quickstart-apps

    with:
      node-version-file: .node-version