JavaScript example: Remove reserved word detection

Reserved word detection as it was implemented in the JavaScript example
grammar had two big downsides:

  1. It required changes in ordering of choices in some rules in order
     not to trigger the detection prematurely. One of the changes was
     already implemented (in the |Statement| rule, see the diff), but
     apparently more were needed (the grammar didn't parse inputs like
     |true| or |function f() {}|). And I'm not 100% sure that would be
     the end of it (maybe deeper structural changes would be needed).

  2. It made error messages confusing. Consider the following example:

       var a = @;

     Instead of reporting:

       Expected ... but "@" found.

     the generated parser reported:

       Reserved word "var" can't be used as an identifier.

     This was because the parser parsed the statement first as
     |VariableStatement| and when this failed, it tried to parse it as
     |ExpressionStatement|, triggering the reserved word detection.

Because of these, I decided to remove reserved word detection from the
JavaScript example grammar.

examples/javascript.pegjs

@@ -146,11 +146,6 @@ SingleLineComment
 
 Identifier
   = !ReservedWord name:IdentifierName { return name; }
-  / name:IdentifierName {
-      error(
-       "Reserved word \"" + name.name + "\" can't be used as an identifier."
-      );
-    }
 
 IdentifierName "identifier"
   = first:IdentifierStart rest:IdentifierPart* {
@@ -993,23 +988,18 @@ Statement
   = Block
   / VariableStatement
   / EmptyStatement
+  / ExpressionStatement
   / IfStatement
   / IterationStatement
   / ContinueStatement
   / BreakStatement
   / ReturnStatement
   / WithStatement
+  / LabelledStatement
   / SwitchStatement
   / ThrowStatement
   / TryStatement
   / DebuggerStatement
-  /*
-   * The following two statements begin with identifiers. They must be at the
-   * very end of the list to prevent triggering reserved word detection in the
-   * |Identifier| rule by keywords the statemets above begin with (e.g. |if|).
-   */
-  / ExpressionStatement
-  / LabelledStatement
 
 Block
   = "{" __ body:(StatementList __)? "}" {