nip-29: restriction for _id_

[kehiy]

I decided to update id details before working on names #1484. here are some restrictions added and more details.

[ismyhc]

This is okay, but I don't really like "." as a character for an id.

I would stick with a-z,0-9,-,_

A Regex would look something like this:

`^[a-z0-9_-]+$`

[kehiy]

@ismyhc I've updated the details. a-z (case-insensitive), 0-9, -, and _. which can support UUID as well.

[ismyhc]

@ismyhc I've updated the details. a-z (case-insensitive), 0-9, - and _. which can support UUID as well.

This works for me.