wp-now: Restore node 18 support

[danielbachhuber]

Minimum supported version was bumped to node 20 in #82 (comment) but we should keep node 18 support for now.

Done is:

• Minimum supported version is 18.
• For any node 20 feature, we throw a warning (or error) if the current version is node 18.

[danielbachhuber]

@sejas Just wanted to let you know that @dmsnell worked through this and put together #90.

As it turns out, wp-now start still works fine on Node 18. wp-now start --blueprint=blueprint.json will crash, but that's documented now in the README. I think this is a fine place for now.

[sejas]

@danielbachhuber , @dmsnell, thank you for the change.

• I created this PR Change nvm node version to 18 #91 to be consistent with the rest of node version changes.