Python: introduce new-CFG facade

Adds 'Cfg.qll' alongside 'AstNodeImpl.qll' in the controlflow internal
package. The facade re-exposes the same API surface as the legacy
'semmle/python/Flow.qll' (ControlFlowNode, BasicBlock, NameNode, CallNode,
AttrNode, ImportExprNode, ImportMemberNode, ImportStarNode, SubscriptNode,
CompareNode, IfExprNode, AssignmentExprNode, BinaryExprNode, BoolExprNode,
UnaryExprNode, DefinitionNode, DeletionNode, ForNode, RaiseStmtNode,
StarredNode, ExceptFlowNode, ExceptGroupFlowNode, TupleNode, ListNode,
SetNode, DictNode, IterableNode, NameConstantNode), but is implemented
on top of the new shared CFG via 'AstNodeImpl.qll'.

The variable-identity predicates ('NameNode.defines', '.uses',
'.deletes', '.isLocal', '.isNonLocal', ...) are one-line bridges to the
underlying AST predicates ('Name.defines', '.uses', '.deletes'),
mirroring the Java pattern.

Re-exports 'EntryBasicBlock' and 'dominatingEdge/2' from the shared
'BB::CfgSig' produced by 'AstNodeImpl.qll', so downstream consumers
(e.g. the SSA adapter) can wire the new CFG into other shared modules
that expect a 'CfgSig' implementation.

This facade is not yet consumed by the dataflow library — that is the
next phase.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: yoff