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.
PEG.js grammar: Remove reserved word detection

This is mostly done for consistency with the JavaScript example grammar,
from which the |Identifier| rule is taken from. See the previous commit
for details.