Conformance tests for updated language spec

[danquirk]

Tracking the test work associated with the spec changes here #1878

• Updates to parameter declarations (3.8.2.2).
• Spread elements in array literals (4.6).
• Updates to contextually typed function expressions (4.9.3).
• Destructuring assignments (4.17.1).
• Updates to contextually typed expressions (4.19).
• Updates to type guards (4.20).
• Destructuring variable declarations (5.1.2).
• Destructuring parameter declarations (6.4).
• Expanded constant enum expressions (9.2).
• const enum declarations (9.4).
• Updates to ambient enum declarations (12.1.4).
• Correction to ambient external module grammar (12.2).

[danquirk]

This should be taken care of before we ship these features.

[yuit]

PR #2735, #2753, #2761, #2773, ##2814, #2785, #2792

[DanielRosenwasser]

Updated the issue to have a task list. @yuit, feel free to check off whichever ones accepted (not pending) PRs have covered.

[yuit]

sweet 🍬 . Thanks @DanielRosenwasser

[yuit]

There already exists conformance test for 12.2