nostr: rework `TagStandard` enum and parsing

[yukibtc]

The TagStandard should be parsed depending on the event kind.

NIP01 tags (p, e and a) must always be parsed, regardless of the event kind.

Also, rework the enum to something like this:

pub enum TagStandard {
    NIP01(TagStandardNip01),
    NIP02(TagStandardNip02),
    // ...
}