Generate more const in ast.ts

[cdietrich]

it would be nice to have consts to be generated for additional stuff in the grammar.

1. keywords

we currently generate

export namespace <grammar> {
    export type KeywordNames =
        | "x"
        | "y"
        | "z";
}

it would be nice to have access to the keywords list at runtime too

export const KeywordNames = ["x", "y", "z"] as const
export type KeywordNames = (typeof KeywordNames)[number]

2. types (am not sure about this)

grammar:

type MyFakeEnum = 'A' | 'B' | 'C';

interface MyIf {
    name: MyFakeEnum;
}

generates

export type MyFakeEnum = 'A' | 'B' | 'C';

export interface MyIf extends langium.AstNode {
    readonly $type: 'MyIf';
    name: MyFakeEnum;
}

having a way to have the possible values of MyFakeEnum available at runtime would be nice too.

[cdietrich]

originally reported here #1979 (comment)

cc @JohannesMeierSE @joao-silveira