Implemented option for parsing text with an alternative entry rule

[spoenemann]

See discussion #1404. This PR adds an option to LangiumParser so one can change the entry rule used for parsing a given text.

[cdietrich]

nice

[Lotes]

Really nice! One concern I have found is scoping. When there is a cross-reference that needs to be resolved, the scope provider writer cannot assume anymore that there are "natural" parents given. Like getContainerOfType would just return undefined, where actually a node is always expected, due to the nature of a translation unit (Like every statement is in a unit. When we only parse a statement we cannot go up to the outer scope). I am wondering how we should reflect this in the documentation...

[spoenemann]

If you use this new feature, you should be aware of the potentially added complexity. Scoping may be completely broken. If you want to support resolving references with a partial model, you need to consider that explicitly in your custom scope provider. The generic one shouldn't have any issues.

[msujew]

Looks good to me 👍