Allow relative `current` releases

Description:

The same way it's possible to define relative LTS versions in node-version (like lts/-1), I want to be able to use relative versions from latest one (like current/-3).

Justification:

I want to support only versions with at least 1 year of maintenance in the foresee. I've seen maintenance dates match for current version, so current/-3 would match up to 3 versions older for current ones, so for example when v24 gets released now in April, -3 would be Node.js v21, not testing on v20 that will be out of maintenance on April 2026.

Additionally, I would want some basic ranges combinations similar to browserlist, so something like >=current/-3 & lts/* when Node.js v24 gets published would resolve to Node.js v22 instead of v21, that's the first LTS newer than current/-3, but maybe that would be in another proposal.

Are you willing to submit a PR?

Yes, I could do that if there's interest.

Start by tracing the existing relative LTS handling for node-version and determine how current/-N should resolve against the latest release. Clarify whether range combinations such as >=current/-3 & lts/* are in scope, then identify the relevant tests and update them to cover release changes; the issue names no files or tests.