Fixes #18300

There are couple of changes here:
1. API change: `ImmutableArray<ISymbolInitializer> MemberInitializers` is changed to `ImmutableArray<IOperation> Initializers`.
2. Implementation changes:
   1. Instead of returning the member initializers as synthesized ISymbolInitializer nodes, we now return member intializers as IAssignmentExpression nodes. This ensures completeness of IOperation tree.
   2. Now we also return the collection intializer expressions within an object creation expression.

Fixes #18115

There are 2 bugs still affecting this area:
1. #18781: IOperation API shape for collection initializer expressions
2. #19276: Missing Field/Property reference expression nodes in object creation initializer node

Ensure that we get a well formed child IOperation tree even for bound nodes for whom IOperation support is not yet implemented

This ensures that the analyzer driver/operation walker is able to find all the descendant IOperation nodes within these not yet implemented features. We should remove the IOperationWithChildren interface once we have designed all the IOperation APIs.

TODO: Implement the VB part.

Fixes #8884

Refactored IVariableDeclaration to IVariableDeclarationGroup, which allows for multiple declared symbols as part of a single declaration.

Since we will try to repalce the logic with existing rewriter, it makes more sense to
minimize the code change here. Also removed the corresponding tests, will fix and add
them back after the implementation is finalized. As result, the contract for
IHasArgumentsExpression.ArgumentsInEvaluationOrder in C# is broken.

Enhance IOperation dumper to dump snippet of syntax and child property names. I have also fixed the existing IOperation tests.

this doesn't replace or change anything yet, just bunch of Operation types for next step.

next step is creating CSharp/VBFactory which return these Operation type and replace callsite that return bound node with this.

after that, I will create xml/generator from Operation.xml.Generated.cs which can produce Operation.xml.Generated.cs

These APIs will be used in autogenerated unit test for Operation tree verification.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=382240.

The TestUtilities project should be our portable unit test helper.  This is part one of rearranging our sources to reflect that reality.  I'm separating it out because it involves a significant code move that
will make the actual rework part harder to review.

Add support for an OperationTreeVerifier - test only OperationWalker to dump operation tree. (#11918)

The dump approach should enable to us to (relatively) easily extend the existing C# and VB binding tests to verify the operation tree.
I have moved the existing IOperation tests to the dump approach and also added couple of additional tests to demonstrate how to write these tests for entire method body operation tree verification.