Adds:
- analyzer/FlowControlScope: Registers a break/continue scope
- analyzer/FlowControlScopes: Registry for break/continue scopes
- ast/LabelableStatement: Base class for AST nodes that can be labelled (loops and case)
- ast/LoopStatement: Base class for AST nodes for loops (have a break and continue label)
- ast/WhileLoop: AST node for a while loop
