- Rename `CSharpRenderingContext` => `CodeRenderingContext`.
- Rename `CSharpCodeWriter` => `CodeWriter`
- Rename `BasicWriter` => `IntermediateNodeWriter`
- Rename `...BasicWriter` => `...NodeWriter`
- Made `CodeRenderingContext` a public abstract API. Left temporary TagHelper pieces in the class since these will be removed soon.
- Moved several `CodeRenderingContext` methods into extension methods. These make use of the Items collection.
- Moved the reporting of missing code target extensions into the `ExtensionIntermediateNode`. This is the integration point of `CodeTarget`s and `CodeRenderingContext`s; therefore, it made more sense existing there.
- Left SetRenderChildren/SetRenderNode as extension methods due to how they're wired up. Aka, to create a `CodeTarget` you need a `CodeRenderingContext`... To Create a `DocumentWriter` you need a `CodeTarget`... To set the render methods on the context you need a `DocumentWriter`.
- Updated tests to utilize the new code renering constructs.
- Moved code generation testability features from `CSharpLoweringPhase` to the `CodeRenderingContext.Items`.
#1043
