Add a build flag for ES6 class syntax

Add a build flag for ES6 class syntax
https://bugs.webkit.org/show_bug.cgi?id=140760

Reviewed by Michael Saboff.

.:

* Source/cmake/WebKitFeatures.cmake:
* Source/cmakeconfig.h.cmake:

Source/JavaScriptCore:

Added ES6_CLASS_SYNTAX build flag and used it in tokenizer to recognize
"class", "extends", "static" and "super" keywords.

* Configurations/FeatureDefines.xcconfig:
* parser/Keywords.table:
* parser/ParserTokens.h:

Source/WebCore:

* Configurations/FeatureDefines.xcconfig:

Source/WebKit/mac:

* Configurations/FeatureDefines.xcconfig:

Source/WebKit2:

* Configurations/FeatureDefines.xcconfig:

Source/WTF:

* wtf/FeatureDefines.h:

Tools:

* Scripts/webkitperl/FeatureList.pm:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@178954 268f45cc-cd09-0410-ab3c-d52691b4dbfc