support extended export syntax

covers
- "[abstract] class"
- interfaces
as per microsoft/TypeScript#3792 (comment) in 2020/12/15

other constructs (microsoft/TypeScript#18628 (comment)) are not yet supported by TypeScript itself

Author: resolritter

common/corpus/declarations.txt

@@ -832,3 +832,22 @@ let a!: b;
       (identifier)
       (type_annotation
       (type_identifier)))))
+
+====================================
+Top-level exports
+====================================
+
+export default abstract class C { }
+export default class C { }
+export class C { }
+export default interface I { } 
+export interface I { } 
+
+---
+
+(program
+  (export_statement (class (type_identifier) (class_body)))
+  (export_statement (class (type_identifier) (class_body)))
+  (export_statement (class_declaration (type_identifier) (class_body)))
+  (export_statement (interface_declaration (type_identifier) (object_type)))
+  (export_statement (interface_declaration (type_identifier) (object_type))))

common/define-grammar.js

@@ -203,7 +203,14 @@ module.exports = function defineGrammar(dialect) {
       export_statement: ($, previous) => prec(PREC.DECLARATION, choice(
         previous,
         seq('export', '=', $.identifier, $._semicolon),
-        seq('export', 'as', 'namespace', $.identifier, $._semicolon)
+        seq('export', 'as', 'namespace', $.identifier, $._semicolon),
+        seq(
+          'export', 'default',
+          choice(
+            seq(optional("abstract"), $.class),
+            $.interface_declaration
+          )
+        )
       )),
 
       non_null_expression: $ => prec.left(PREC.NON_NULL, seq(