Specify maximum bech32 decode size in NIP-19

[hoytech]

As far as I can tell, the defacto standard is 5000, for example:

https://github.com/nbd-wtf/nostr-tools/blob/1b955bdbcb7775833930836930b44e88949101cd/nip19.ts#L24

If different implementations use different values for this, they will not be able to decode eachother's nevents/nprofiles/etc.

[staab]

#2264

[hoytech]

Thanks, LGTM!