Improve type inferrence of data type rules

The idea came from the discussion. Currently, ON returns string: 'on'; infers

export type ON = string;

Instead, we could infer it like

export type ON = 'on'

[pluralia]

Not a big fan of reintroducing enum rules to the grammar.

Okay, adding enums is not what we want. However, I’d anyway expect that MyEnum returns string: 'on' would generate a type on for MyEnum and not just a string. It’s like a enum with the only one element, and if for MyEnum returns string: 'on' | 'off' we generate a type 'on' | 'off', why not do the same for the only one type alternative?

[pluralia]

The case described in https://github.com/langium/langium/discussions/720 is where the user expects an enum behavior of a datatype rule and I can imagine a lot of cases where it is a good idea, so what about introducing an enum datatype rule?

@dhuebner, yes, I like this idea. For example, this

MyEnum returns string: ON | OFF;
enum MyRealEnum: ON | OFF;
ON returns string: 'on' ('and' 'on')*;
OFF returns string: 'off'+;

will generate

export type MyRealEnum = ON | OFF
export type MyEnum = string

So, addition a keyword enum restricts to use only data type rules in a union, and we can infer type like a union of these data type rules without tries to evaluate a pattern.