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

[MikeMcC399]

• follows on from ci: update to Node.js 22 LTS #21 (comment)

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/test.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.

cc: @AtofStryker