Short name requirements

The `shortName` field is a little inconsistent right now, and a few stand out as not particularly short and with varying combinations of capitalization, spaces, and dashes. (E.g. "Telos EVM", "CELO", "Ontology Testnet", "aurora-testnet", "tpalm" with "t" prefix indicating testnet.)

I couldn't find guidelines for the correct or expected formatting, and I think it would be beneficial to standardize it a little more.

I would suggest at the very least a requirement of no spaces. On top of that, it should try to be reasonably short, thus generally not the same as the `name` field. The vast majority right now are lowercase and I personally think that's preferable.

I think it is implicit but could be made explicit, that the short name must be unique.

I suppose these requirements could be mentioned in the "new chain" issue template.