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/AST.cpp

@@ -4045,6 +4045,8 @@ Completion ImportStatement::execute(Interpreter& interpreter, GlobalObject& glob
     return interpreter.vm().throw_completion<InternalError>(global_object, ErrorType::NotImplemented, "'import' in modules");
 }
 
+FlyString ExportStatement::local_name_for_default = "*default*";
+
 // 16.2.3.7 Runtime Semantics: Evaluation, https://tc39.es/ecma262/#sec-exports-runtime-semantics-evaluation
 Completion ExportStatement::execute(Interpreter& interpreter, GlobalObject& global_object) const
 {
@@ -4111,14 +4113,14 @@ void ImportStatement::dump(int indent) const
     }
 }
 
-bool ExportStatement::has_export(StringView export_name) const
+bool ExportStatement::has_export(FlyString const& export_name) const
 {
     return any_of(m_entries.begin(), m_entries.end(), [&](auto& entry) {
         return entry.export_name == export_name;
     });
 }
 
-bool ImportStatement::has_bound_name(StringView name) const
+bool ImportStatement::has_bound_name(FlyString const& name) const
 {
     return any_of(m_entries.begin(), m_entries.end(), [&](auto& entry) {
         return entry.local_name == name;