atproto/lexicon:Allow unions as definitions

[wdantuma]

This PR adds union as a type allowed in definitions so it behaves the same as the indigo implementation does ( goat )

[notjuliet]

I'll share what I've been told on why that might not be a desired behavior:

basically you can't really determine the link between app.bsky.feed.post#foo and app.bsky.feed.post#main if main is a union type linking to foo, was foo the result of creating an object with main? or the result of itself? the $type field cannot convey this info

[wdantuma]

I understand that is a problem, and disallowing unions for main would not be a problem at all, actually I will update the PR for it, but not being able to reuse a predefined union is ( for me ) a big issue for lexicon reuse, I ( and I guess more people ) have a lot of use cases with large unions (more than 10 types), and always needing to copy their definitions in types that use the union would be very developer unfriendly.

[wdantuma]

Added test to check if union as main definition is still invalid

[wdantuma]

@bnewbold, @devinivy Is this a correct fix for this issue ?

[bnewbold]

I'm still mulling this over. I expect us to have it settled in the next month or two.