Split out codegen from Ecsact CLI

[zaucy]

We still want codegen to be a subcommand of the Ecsact CLI, but it has become apparent that Ecsact Codegen is useful as a standalone library in our other repositories.

Something that doesn't require file IO may be ideal. Then we can run it in the browser with a wasm compiled version. It could be the users responsibility to parse and interpret the ecsact files and have the codegen API read the available declarations.