feat(lex-cli): loosen the validation of the root document

[prenaissance]

This PR allows extra properties to be specified for lexicons at the root of the declaration file. Everything under the deps field still remains strictly validated.

Motivation

I've been trying to experiment with json schemas for the lexicon declarations in my project, and I realized that the codegen cli does not allow me to compile declarations with extra properties (including $schema):

Invalid lexicon /home/prenaissance/<some_path>/atproto/lexicons/comment.json
Issues at :
 - Unrecognized key(s) in object: '$schema'

The schema validators were made to use ZodSchema::strict() in #1088, in response to the issue from #1080, but allowing extra properties at the root level of the document would do no harm.