Backwards compatibility: what do we promise, and to whom?

[joostfarla]

A few proposals here add to the ruleset format, like wider use of aliases (#2) and a new name for
formats (#5). Before we pick any of them up, I think we should agree on what we promise about
compatibility, since the answer shapes how we design them.

There are (at least) two directions:

1. Existing rulesets in new tools. Every ruleset that works in Spectral today keeps working
   in OpenLint and stays valid under the spec.
2. New rulesets in existing tools. A ruleset that uses something new still loads in
   Spectral, Vacuum, and anything else that reads the format.

I'd make the first a hard rule. I don't think we can promise the second: Spectral and Vacuum both
reject keys they don't know, so any addition breaks them.

Are there any other concerns regarding backwards compatibility?