Add generator for API modules

Since it can take a long time to implement new Neovim API features and it can be error prone, it should be generated. At first this was avoided to provide a higher quality API compared to automatically generated one, but it is very time consuming and hard to maintain. This generation can be done whenever Neovim is updated and updates API can be released. Future improvement could be adding a module which automatically generates API for users oft he library.

The issue names no files, tests, or entry points. Start by mapping the existing Neovim API modules and how Neovim updates are currently incorporated; done should be a repeatable process that generates and releases updated API modules.

Written by the indexing model from the issue text.