Fix parsing error in exponent expressions with unary left-hand sides.

Esprima (correctly) rejects expressions like -1**2.
However, expressions like (-1)**2 are valid but still rejected.
This commit fixes this issue by identifying when the left operand
is parenthesized.

Fixes jquery/esprima#1981

src/Esprima/JavascriptParser.cs

@@ -1861,10 +1861,14 @@ private Expression ParseUnaryExpression()
 
         private Expression ParseExponentiationExpression()
         {
-            var startToken = _lookahead;
-
-            var expr = InheritCoverGrammar(parseUnaryExpression);
-            if (expr.Type != Nodes.UnaryExpression && Match("**"))
+            var startToken = _lookahead;
+
+            var isLeftParenthesized = this.Match("(");
+            var expr = InheritCoverGrammar(parseUnaryExpression);
+
+            var exponentAllowed = expr.Type != Nodes.UnaryExpression || isLeftParenthesized;
+
+            if (exponentAllowed && Match("**"))
             {
                 NextToken();
                 _context.IsAssignmentTarget = false;