Add regression test. (microsoft#29433)

Author: DanielRosenwasser

tests/cases/fourslash/server/documentHighlightsTypeParameterInHeritageClause01.ts

@@ -0,0 +1,6 @@
+/// <reference path="../fourslash.ts" />
+
+////interface I<[|T|]> extends I<[|T|]>, [|T|] {
+////}
+
+verify.rangesAreDocumentHighlights();