Add types declaration

[pluralia]

• Adds interface and type syntax
• Adds ReturnType & adds it to the scope for cross-references
• Removes properties lifting by replacement interfaces with types in the AST (closes #408)
• Adds references in types as @ and arrays as [] (for example, @MyType[] means an array of references on MyType)
• Adds types in the AST reflection
• Sorts the AST: interfaces and types / fields / fields types / container types / cases in reflection

[spoenemann]

Thanks, @pluralia! At first glance this looks good. I'll have a more in-depth look later, here are a few initial remarks.

[msujew]

This is looking fantastic, thanks 👍

I noticed that highlighting of return types for terminal rules does no longer work as expected. You should replace this code:

https://github.com/langium/langium/blob/e3145e483ab2e9afe9554bb8d6c899d492b6d34a/packages/langium/src/grammar/langium-grammar-semantic-token-provider.ts#L30-L35

With this:

} else if ((isParserRule(node) && isDataTypeRule(node) || isTerminalRule(node)) && node.type?.name) {
  acceptor({
    node: node.type,
    feature: 'name',
    type: SemanticTokenTypes.type
  });
}

[spoenemann]

Let's ship this! We need to work on a few basic validations, then release v0.3.0 is ready to go.