Port generator to Langium

[lars-reimann]

Additional material:

• https://www.typefox.io/blog/code-generation-for-langium-based-dsls
• https://langium.org/tutorials/generation/

Tasks:

• basic generator
• remove varargs from tests
• filter Statements without effect (related to Port partial evaluator to Langium #603)
• wildcard imports
• verify special names when importing from pipeline/test inside the same package (see test generation/imports)