[4.35+] Refactor ConditionalExpression and SwitchExpression by making them implement INumericChoiceContext

JLS 5.6 classifies `SwitchExpression` and `ConditionalExpression` as constructs in numeric choice context (when all result expressions are numeric) The computation of result type, their polyness or not all are all similarly defined. Essentially a switch is multi-way-conditional.

As such there is enormous room for code sharing by hoisting contract and default behavior to a new interface type say `INumericChoiceContext` that both switch expressions and ternary operators implement.

Comparing `org.eclipse.jdt.internal.compiler.ast.SwitchExpression.resolveType(BlockScope)` and `org.eclipse.jdt.internal.compiler.ast.ConditionalExpression.resolveType(BlockScope)` it is possible for the latter to be as minimal as the former.