Snippet refactoring 2

[DoctorKrolic]

Follow up to #74712

2 changes here:

1. Do not pass SnippetContext around by in reference. The only actual data, that SnippetContext holds is 1 reference to a SyntaxContext. Therefore passing it around by in doesn't win anything, but adds unnecessary cognitive overhead when reading that code
2. Since SnippetContext already holds a SyntaxContext instance, all async work of getting syntax tree and semantic model from the document has already been done. Therefore it isn't practical to keep this path async. I remember there was a discussion, that semantic snippets engine is a potential candidate for becoming public API. And since there is no way SyntaxContext become public, I additionaly exposed SemanticModel as a public property of SnippetContext. That way a potential API user can get everything they need without having to deal with any async stuff:

• Syntax tree: context.SemanticModel.SyntaxTree
• Syntax root: context.SemanticModel.SyntaxTree.GetRoot()
• Semantic model: context.SemanticModel
• Compilation: context.SemanticModel.Compilation

[DoctorKrolic]

@akhera99 PTAL

[akhera99]

Looks good, thanks for the contribution!