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

Esprima (correctly) rejects expressions like -1**2. - jquery/esprima#2070
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

@@ -1860,10 +1860,14 @@ 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;