Implement Lexicon fetching & type generation

[bdbch]

As @skiniks already started working on this I wanted to add an issue to track this feature.

Tsky should not rely on types provided by the atproto api package as we want to phase away from it in the future. For that we need to fetch the Bluesky lexicons by ourselves.

After that we need a way to generate typings from said lexicons (plus allow users to generate typings from their own custom lexicons via a CLI).

• Downloading Lexicons (feat(types): add lexicon type generation #23)
• Generating Typescript Definitions (feat: add @tsky/lex-cli for generating typescript definitions #28)

[shuuji3]

This issue can be closed since #28 has already implemented both functions.