Implement typechecker

[DylanSp]

Reference https://github.com/DylanSp/faust-lang/issues/2

No type inference - all variable and function declarations must be explicitly typed.

[DylanSp]

If I implement the idea from https://github.com/DylanSp/faust-lang/issues/28 about parametrizing AST nodes with type info - the typechecker could, instead of returning "successfully typechecked" vs. "type error(s)", include type errors in the annotated AST:

type Type = TypeBool | TypeInt | TypeError;
interface TypeError {
  tag: "Error";
  location: ast.Location;
  message: string;
}

See https://matklad.github.io/2023/08/17/typescript-is-surprisingly-ok-for-compilers.html.

• TypeError would be equal to any other type when typechecking to prevent cascading failures.
• Reporting if typechecking was successful would then just mean checking if there are any TypeError's in the annotated AST.