Add schema validation to Cursorless api boundary

[pokey]

For all places where the Cursorless extension ingests data from the outside world, we should validate against a schema. Here are a few such places:

• [ ] Cursorless command arguments (both primary cursorless.command action and others, eg experimental keyboard)
• [ ] Custom snippets (and built-in snippets)
• [ ] Settings
• [ ] other places?

We'd like to define these schemas using zod. This library will automatically allow Typescript to infer types. We can start by using ts-to-zod to generate initial schema

For custom snippets and settings, we can look into automatically generating json schema using zod-to-json-schema so that we have a single source of truth