Generate isType function for primitive datatype rules

[gfontorbe]

closes #990

The idea is to generate checker functions for datatype rules through langium-cli.
This only applies on datatype rules that are only composed of strings or terminals. I also had to slightly touch the type inference to provide the regex for terminals.

Example:

A returns string:
    'a';

AB returns string:
    A | 'b';

ABC returns string:
    AB | ID;

will generate the following functions in the generated ast.ts file:

export function isA(item: string): item is A {
    return item === 'a';
}

export function isAB(item: string): item is AB {
    return isA(item) || item === 'b';
}

export function isABC(item: string): item is ABC {
    return isAB(item) || /[_a-zA-Z][\w_]*/.test(item);
}

[spoenemann]

Could you run npm run langium:generate in the root and check in the resulting changes?

[msujew]

I like it, thanks!

Can we also include generated checks for number, bigint and Date types? I think that would complete the feature on this part.

[msujew]

Looking fairly good. I have a few minor improvements.

[msujew]

I've simplified the generator tests a bit further. Looks good to me now 👍