Remove mixed enums; add ast-grep

[just-be-dev]

@manzt pointed out some broken generation in the python client in #78. After investigating this for a bit, it looks like it's due to mix enum styles in the rust code.

Here's a basic example

pub enum WindowSize {
    States(WindowSizeStates),
    Size { width: f64, height: f64 },
}

My typescript generation code outputs the following for this case

export type WindowSize = WindowSizeStates | {
  height: number;
  width: number;
};

Typescript has the advantage of allowing for anonymous objects in unions. What should the second object be in python? Currently my code is spitting out a class called undefined because I don't have a name for it.

As with #108, often the solution is to simplify the rust data structures to simplify the generation. The final structure I ended up with for the above example is

pub enum WindowSize {
    States(WindowSizeStates),
    Size(Size),
}

All of the mixed enums had to do with size, so unifying the enums actually reduced some duplication anyway.

---

This PR also introduces ast-grep to the codebase. I've been using ast-grep significantly more at val.town and it's quickly becoming one of my go-to tools for code quality. I added a no-mixed-enums rule which ensure this pattern doesn't get reintroduced in the future.