Add validation of implicit `infers` usage

We already validate incorrect explicit usage of returns and infers in #438 – we should validate implicit usage too.

For example, a Definition rule can be defined without a return type, what implicitly means Definition infers Definition.

Definition:
    'def' name=ID ('(' args+=DeclaredParameter (',' args+=DeclaredParameter)* ')')? ':' expr=Expression ';';

If now interface Definition will be declared, we should validate, that the Definition rule has the return type Definition as Definition returns Definition.

Definition returns Definition: ...;

interface Definition {
    name: string
    expr: Expression
    args: DeclaredParameter[]
}

Depends on #440.

[msujew]

@montymxb I believed this behavior was already added by https://github.com/langium/langium/pull/461. But I agree having a quick fix for this would be great!

[dhuebner]

We need to fix scoping anyways, currently in your example returns Definition will link to Parser rule Definition 😃