Actions can rewrite declared types

Actions can rewrite declared types, effectively erasing/changing their previously declared properties. This leads to an inability to reason about declared types as a fixed source of information about the resulting AST that is generated.

An example grammar in Langium that causes this:

// 1. define some interface
interface A {
    val: string
}

// X inherits correctly from A via a code action, leaving A unchanged
X: 'x' {A} val=ID;

// but Y redefines A entirely
Y: 'y' {infer A} q='broken';

assumes the ID terminal is declared

This generates the following interface, which is not correct:

export interface A extends AstNode {
    q: 'broken'
}

Langium version: 0.3.0 (dev)

Instead:

• declared types should be immutable, their properties should not be open to changing, erasure, or addition.
• validation should indicate something like A is a declared type and cannot be inferred
• optional: add a code action to rewrite {infer A} to {A} (could potentially be in a separate PR)

[dhuebner]

@montymxb

A is a declared type and cannot be inferred

I would like to propose: A is a declared type and cannot be redefined

[spoenemann]

(Note: Code actions are an LSP request type, which is totally unrelated to actions in the grammar)