LibJS: Make parsing import and export entries follow the spec

The big changes are:
- Allow strings as Module{Export, Import}Name
- Properly track declarations in default export statements

However, the spec is a little strange in that it allows function and
class declarations without a name in default export statements.
This is quite hard to fully implement without rewriting more of the
parser so for now this behavior is emulated by faking things with
function and class expressions. See the comments in
parse_export_statement for details on the hacks and where it goes wrong.

Userland/Libraries/LibJS/Parser.h

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020, Stephan Unverwerth <s.unverwerth@serenityos.org>
- * Copyright (c) 2021, David Tuin <davidot@serenityos.org>
+ * Copyright (c) 2021-2022, David Tuin <davidot@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -214,7 +214,7 @@ private:
 
     RefPtr<BindingPattern> synthesize_binding_pattern(Expression const& expression);
 
-    Token next_token() const;
+    Token next_token(size_t steps = 1) const;
 
     void check_identifier_name_for_assignment_validity(StringView, bool force_strict = false);
 
@@ -225,7 +225,9 @@ private:
 
     bool parse_directive(ScopeNode& body);
     void parse_statement_list(ScopeNode& output_node, AllowLabelledFunction allow_labelled_functions = AllowLabelledFunction::No);
-    void parse_assert_clause(ModuleRequest& request);
+
+    FlyString consume_string_value();
+    ModuleRequest parse_module_request();
 
     struct RulePosition {
         AK_MAKE_NONCOPYABLE(RulePosition);