Rename GPT3CompletionModel to something more accurate

[falquaddoomi]

Near the creation of the tool, the revision model that integrates with an external LLM has been named GPT3CompletionModel. Since that choice was made, OpenAI has released various new versions of GPT, e.g. GPT-3.5, GPT-4, GPT-4o, etc. Also, since PR #60, the model has been updated to work with other LangChain-compatible providers, e.g. Anthropic's Claude, that don't refer to their LLMs as GPTs at all.

It would be good to rename this class to something that captures that it works with any supported provider/LLM, and that it's the main class that's used for revisions; unlike the other descendants of ManuscriptRevisionModel, it isn't just for testing. Perhaps LLMCompletionModel could work?

Since https://github.com/manubot/manubot references GPT3CompletionModel in several places, we'll have to have a PR in that repo as well that introduces the renamed class.