Semantic snippets: Implement do snippet

[DoctorKrolic]

Part of #64144 (don't forget to check the item when merging)

[DoctorKrolic]

@CyrusNajmabadi Regardless your request on extracting constants: I tried to preserve layering and separated identifiers into 2 categories: those which can theoretically be reused in VB (e.g. cw) and those which are C# only (keywords and so on). As part of that work I moved some properties up to C# layer (e.g. for else identifier since it is clearly a C# keyword and in VB it would be Else, same with description). Also since I touched most files in snippets folder anyway, I refactored classes in it a bit by making importing constructors primary and sealing what was still unsealed

[DoctorKrolic]

@CyrusNajmabadi PTAL

[CyrusNajmabadi]

nit: int eh future, it would nice to use generics (with teh base type) to make it so that all this is strongly typed.

[DoctorKrolic]

In this particular example this would just move the hard cast one layer above:

roslyn/src/Features/Core/Portable/Snippets/SnippetProviders/AbstractConditionalBlockSnippetProvider.cs

Lines 21 to 26 in e6c6877

	protected override ImmutableArray<SnippetPlaceholder> GetPlaceHolderLocationsList(SyntaxNode node, ISyntaxFacts syntaxFacts, CancellationToken cancellationToken)
	{
	if (ConstructedFromInlineExpression)
	return [];
	var condition = GetCondition(node);

GetPlaceHolderLocationsList is from the AbstractSnippetProvider class, which is a base class for all snippets. And even if we try to make AbstractSnippetProvider generic as well, we will still need to hard cast here (see how mainChangeNode is just found via annotation, the cast would be here):

roslyn/src/Features/Core/Portable/Snippets/SnippetProviders/AbstractSnippetProvider.cs

Lines 110 to 123 in e6c6877

	var mainChangeNode = reformattedRoot.GetAnnotatedNodes(FindSnippetAnnotation).FirstOrDefault();
	Contract.ThrowIfNull(caretTarget);
	Contract.ThrowIfNull(mainChangeNode);
	var annotatedReformattedDocument = documentWithIndentation.WithSyntaxRoot(reformattedRoot);
	// All the TextChanges from the original document. Will include any imports (if necessary) and all snippet associated
	// changes after having been formatted.
	var changes = await annotatedReformattedDocument.GetTextChangesAsync(document, cancellationToken).ConfigureAwait(false);
	// Gets a listing of the identifiers that need to be found in the snippet TextChange
	// and their associated TextSpan so they can later be converted into an LSP snippet format.
	var placeholders = GetPlaceHolderLocationsList(mainChangeNode, syntaxFacts, cancellationToken);

[CyrusNajmabadi]

Yup. mkaing these changes in #72892

[CyrusNajmabadi]

Thanks!