* Add support for Optional Chaining
* Add grammar error for invalid tagged template, more tests
* Prototype
* PR feedback
* Add errors for invalid assignments and a trailing '?.'
* Add additional signature help test, fix lint warnings
* Fix to insert text for completions
* Add initial control-flow analysis for optional chains
* PR Feedback and more tests
* Update to control flow
* Remove mangled smart quotes in comments
* Fix lint, PR feedback
* Updates to control flow
* Switch to FlowCondition for CFA of optional chains
* Fix ?. insertion for completions on type variables
* Accept API baseline change
* Clean up types
* improve control-flow debug output
* Revert Debug.formatControlFlowGraph helper
