Refactor Statement and Expression node parent classes

[barnjamin]

Since Statements and Expressions both inherit from BaseNode some issues arise in accessing attributes that are not defined in one or the other. We can change the inheritance structure to split the different Node types for better type information and prevent issues with inferring attribute availability.

See this comment for more: https://github.com/tinymanorg/tealish/pull/27#pullrequestreview-1206813701