LibJS: Fix syntax error for arrow function non-decl variable assignment

A regression was introduced in dc9b4da where the parser would
incorrectly parse the assignment of arrow functions to (non-declaration)
variables. For example, consider:

    a = () => {}

Because the parser was aware of default parameters, in
try_parse_arrow_function, the equals sign would be interpreted as a
default argument, leading to incorrect parsing of the overall
expression. Also resulted in some funny behavior
(a = () => {} => {} worked just fine!).

The simple fix is to only look for default parameters if the arrow
function is required to have parenthesis.

Libraries/LibJS/Parser.cpp

@@ -295,7 +295,7 @@ RefPtr<FunctionExpression> Parser::try_parse_arrow_function_expression(bool expe
         } else if (match(TokenType::Identifier)) {
             auto parameter_name = consume(TokenType::Identifier).value();
             RefPtr<Expression> default_value;
-            if (match(TokenType::Equals)) {
+            if (expect_parens && match(TokenType::Equals)) {
                 consume(TokenType::Equals);
                 default_value = parse_expression(0);
             }

Libraries/LibJS/Tests/arrow-functions.js

@@ -4,6 +4,9 @@ try {
     let getNumber = () => 42;
     assert(getNumber() === 42);
 
+    getNumber = () => 99;
+    assert(getNumber() === 99);
+
     let add = (a, b) => a + b;
     assert(add(2, 3) === 5);