Improve developer ergonomics for precompiled validator usage

[greg-nagy]

Background

In #796 we moved to precompile the validator functions so we can remove the unsafe-eval CSP policy but it happened in a bit of a rush.

Purpose

The usage of ajv is not really developer-friendly, and it would be good to be able to use it more conveniently.

Outline for the solution

• move all the schema definitions into a separate folder
• write code that imports all of them and
  • adds them to an ajv instance
  • compiles the validators in esm modules
  • writes the compiled output into a file
• maybe
  • generates type guard functions and exports them, so they can be used throughout the codebase
  • keep the schema files next to the implementation code

References

• ajv standalon usage docs https://ajv.js.org/standalone.html
• ajv standalone esm code generation issue: Option to generate ESM for standalone validators ajv-validator/ajv#1523
• ajv standalon example thread: Concrete example of using standalone at runtime ajv-validator/ajv#1831
• ajv esm compile docs https://ajv.js.org/guide/environments.html#cjs-vs-esm-exports
• this project uses ajv in a similar manner described above gajus/table
  • https://github.com/gajus/table/blob/master/package.json#L73 - compile npm script
  • https://github.com/gajus/table/blob/master/src/types/generated/validators.d.ts - typing