Emit diagnostics for semantic validations.

[sam-goodwin]

Implement a TypeScript Language Service Plugin which adds domain-specific diagnostics.

https://github.com/microsoft/TypeScript/wiki/Writing-a-Language-Service-Plugin

[sam-goodwin]

This will have to be an ES build plugin because a language service plugin can not fail the build since they don't run during tsc.

[sam-goodwin]

Actually, nevermind, we can use ts-morph! https://github.com/nonara/ts-patch#altering-diagnostics

[sam-goodwin]

Nevermind (again!), according to https://github.com/nonara/ts-patch#notes

This alters diagnostics during emit only. If you want to alter diagnostics in your IDE as well, you'll need to create a LanguageService plugin to accompany your source transformer

We will have to do a language service plugin. In order to share validation logic between the language service plugin, I propose we have two separate compile passes:

• validate
• transform

They will both use the FunctionlessChecker which centralizes all of the "queries" we wish to ask of the AST. It extends TypeChecker to inherit all of TS's type checking queries.