- Previously we'd special case `@section` at code generation time; now we transform the directive into an IR node.
- Changed the expectations of `DefineSection` to not take in a section writer. It's now expected to modify what `Write`, `WriteLiteral` etc. write to when inside of the lambda. This is done today in TagHelpers via `StartTagHelperWritingScope`.
- Updated baseline files to reflect new `DefineSection` expectations.
- Updated IR tests since we no longer leave around `DirectiveIRNode`s.
#901
