mostly cleaning up details in the type declarations and the code.

main changes are:

• Expression.variant hasn't for a while been capable of being a string yet we where checking in some cases and the linter did not pick on that.
• since Prettier V3 the function getNode returns null if there is no value therefore checks for undefined are unnecessary.
• saving groupId in a constant instead of fetching it from a generated group object. (very similar performance but we avoid using a non-null assertion which don't look very nice)
• using some clever typescript magic to get a union of all the types within a namespace instead of naming all the possible art nodes.