Refactor and simplify ExpressionSyntaxRewriter

[PhenX]

This class has become huge, it is time to simplify it.

I suggest this :

• Create EnumMethodExpander.cs - extract TryExpandEnumMethodCall and CreateMethodCallOnEnumValue
• Create SwitchExpressionRewriter.cs - extract VisitSwitchExpression and ReplaceVariableWithCast
• Create NullConditionalRewriter.cs - extract logic for null coalesce operators
• Simplify main file and delegate to these new classes