The `langium@3.1.0` changes broke existing grammars

[Yokozuna59]

Langium version: 3.1.0
Package name: langium

Steps To Reproduce

1. Create a Hello World example using the generator.

2. Modify the grammar to has a rule with multiplicity without assigning to any value:

   grammar HelloWorld
   
   entry Model:
   +  	ID*
   	(persons+=Person | greetings+=Greeting)*;

Link to code example:

The current behavior

The changes after #1401 broke the existing working grammars.

The expected behavior

No errors.

[spoenemann]

The change is as expected. What is your use case for a multiplicity without an assignment?

I your second example, I'd recommend to create a data type rule:

Newlines returns string: NEWLINE+;

and then use Newlines? instead of NEWLINE*.

We could consider making an exception for terminal rules, though.

[msujew]

@spoenemann I'd argue that we should make an exception for terminal and data type rules.

[msujew]

The latest release (3.1.1) includes a fix for this.

[spoenemann]

What should we do with the milestone assignment in such a case? Assign to v3.1.0 or v3.2.0? No milestone?

I think assigning to v3.1.0 would be most helpful.

[msujew]

I agree, I've added it to the v3.1.0 milestone.